Elegance is as important in scientific design as it is in art and architecture, chemical engineer Nicholas Kotov believes. Sitting in his austere office at the University of Michigan, in Ann Arbor, he shows off a swatch of black cotton; in heft and feel it’s similar to a soft, lightweight dress shirt. But Kotov has transformed the fabric into a biosensor and an electrical conductor simply by dipping it into a solution of carbon nanotubes, antibodies, and a polymer.
Individual, well-formed carbon nanotubes are highly conductive, which makes them promising for applications such as battery electrodes and microprocessors. If molecules such as antibodies are anchored to their surface, they can also serve as very sensitive chemical detectors: when an antibody binds to its target, the nanutobe’s electrical properties are measurably altered. But nanotubes tend to clump together, which prevents them from functioning individually. That seriously degrades their electronic properties, says Kotov.
There are ways around this problem: nanotubes can be painstakingly laid down, one by one, using methods that involve days of solution processing followed by photolithography, or the tubes can be sprayed onto a flat surface in alternate layers with a conductive polymer, which prevents clumping. But Kotov found that this type of layer-by-layer assembly can be further simplified for a complex three-dimensional surface such as a cotton thread: the tangle of fibers provides a structural template that allows him to simply dip the thread into a solution containing both the polymer and the tubes. Glued to the thread by the polymer, the nanotubes form a net with good electrical properties, the tubes overlapping but well spaced.
The method results in a sleek, powerful, and much more wearable alternative to complex intelligent textiles that incorporate heavy, bulky optical fibers or corrosion-prone metal wires. While Kotov is exploring a number of possible applications for these textiles, the most important, he says, would be as biosensors to keep people safe. They could be used to spot blood loss in soldiers on remote patrols or to detect airborne allergens or pathogens such as influenza. And the threads are cheap and sensitive enough for possible use in factories or stores, or even in the home–for example, to test an iffy batch of peanut butter for toxins.
A Quick Dip
In Kotov’s lab, graduate student Jian Zhu mixes commercially available single-walled nanotubes and a polymer called Nafion into ethanol, which prevents the components from sticking together. The Nafion glues the nanotubes to the cotton, but that’s not all it does. Nafion, a long, conductive molecule composed primarily of carbon, acts like a tiny spring, allowing each nanotube some measure of independent movement. This mechanical property, which is critical for biosensing, also allows the cotton to maintain its softness and give: you wouldn’t want to wear a shirt coated in stiff epoxy.
Zhu snips a length of ordinary cotton thread from a spool and uses a pair of tweezers to submerge it in the inky-black solution. After it sits for two minutes, he fishes out the thread and uses a binder clip to hang it up to dry inside a lab hood, a process that can be shortened to only a few minutes with a hair dryer. The electrical resistance of the thread is optimized, Kotov has found, when it has been dipped about 10 times.
In the group’s student office, Zhu demonstrates the electronic properties of a finished nanotube thread, which is indistinguishable from ordinary black cotton. He attaches it to the electrical contacts on a white light-emitting diode using ordinary solder, then draws the ends of the thread through the positive and negative clips on a power source. He turns the power source up to three volts, and the light shines brightly.