Water utilities have a Goldilocks problem: If they don't add enough chlorine, nasty bacteria that cause typhoid and cholera survive the purification process. Too much chlorine produces disinfection byproducts such as chloroform, which increase cancer risks. The amount of chlorine needs to be “just right” for safe drinking water.
The Environmental Protection Agency regulates how much of the disinfection byproducts, including those known as trihalomethanes, are allowed in our drinking water. But if water utilities want to monitor and control their own trihalomethane levels, they have to send off samples and wait weeks for analysis by an EPA-qualified lab.
Working with Parker Hannifin, Sandia National Laboratories combined basic research on a form of carbon with a unique microsensor to make an easy-to-use, table-top tool that quickly and cheaply detects extremely low levels of each trihalomethane: chloroform, bromoform, bromodichloromethane, and dibromochloromethane.
The original goal of the project was to make a hand-held chemistry lab, like a tricorder, to detect airborne hazardous chemicals, including chemical weapons. A principal component of this lab-on-a-chip was a surface acoustic wave sensor (SAW), which works by vibrating a wave along a quartz sheet. By measuring how the wave changes on the SAW device, researchers can tell how many chemicals are sticking to the quartz surface.
However, the quartz surface isn't very sticky, which limits its sensitivity. This is where a special carbon coating comes in. Nanoporous carbon consists of stacked nanofragments of graphene sheets, engineered with lots of nooks and crannies where chemicals can lodge. Unlike carbon nanotubes or graphene with similar molecularly “sticky” surfaces, nanoporous carbon can be grown onto almost anything, including SAW devices.
This growth process, known as pulsed laser deposition, involves zapping graphite with a laser at room temperature. The liberated carbon atoms fly through a vacuum chamber to coat the SAW sensor in a uniform and reproducible manner. By adding a little bit of an inert gas to the vacuum chamber, the density and total surface area of nanoporous carbon coatings can be varied from very fluffy to as solid as pure graphite.
For the SAW sensors, nanoporous carbon with a middling density turns out to be best.
Initially, Sandia's newest and smallest SAWs used higher frequency vibrations with more advanced microelectronics. However, they were also more expensive, harder to make, and less reliable. Using larger devices, roughly the dimensions of a Tic Tac, that were state-of-the-art in the '90s, it was easy to apply the coating — which increased the sensitivity a thousand times more than decreasing the size — and dramatically decreased the cost of the device. The latest version of the analyzer can automatically monitor individual trihalomethane levels every hour.
Other potential uses for nanoporous carbon coated SAW sensors include detecting homemade explosives, contaminants in air and water, and almost any volatile or semi-volatile organic compound. Researchers are also exploring using nanoporous carbon for battery anodes.