Intelligent Machines

Cheap Chemical Sensors

Electronic “noses” made from printed electronics could detect toxic chemicals inexpensively.

Technology already exists that can sniff out chemicals in the air and water – but the detecting devices are expensive, limiting their use. Now Vivek Subramanian, electrical engineering professor at the University of California, Berkeley, has made arrays of sensors cheap enough that they could be widely distributed for monitoring toxins in the environment.

The goal is to “identify environmental problems before they become severe, then react to them,” says Subramanian. “One of the major requirements, if we want to do this, is ultra-low cost,” he says. Subramanian makes his array of inexpensive chemical sensors using organic semiconductors and inkjet printing technology. The first generation of his devices, which would still rely on costly silicon-based technology to process signals from the sensors, would run about 30 cents a piece, Subramanian estimates. That’s a bargain compared with several hundred dollars for today’s sensors, he says. Subramanian reported on his work at the Materials Research Society meeting in Boston this week.

Organic transistors tend to degrade, especially when exposed to air, chemicals, or water. Yet this reactivity can also make them good sensors. For one thing, different chemicals affect the rate at which the sensors degrade. Subramanian’s innovation is to use an array of different organic semiconductors, each responding slightly differently to different chemicals. The signals from this array then create a distinctive pattern – a sort of electronic fingerprint of a particular chemical.

The idea of using an array for chemical sensing, first proposed in the late 1970s, mimics the behavior of the human nose, which can recognize a wide range of different chemicals without having sensors for detecting each one. “What Subramanian is doing is borrowing this very clever idea and applying it to organic transistors, which definitely makes sense,” says Luisa Torsi, professor at the University of Bari in Italy. Torsi was one of the first to recognize the potential of organic transistors as sensors and to develop the type of transistor at the heart of Subramanian’s device.

Subramanian prints the different kinds of organic semiconductors in arrays using the multiple nozzles of a inkjet printer. So far, he has only produced arrays of transistors using five different semiconductors – still more than enough to detect the difference between good and spoiled wine, for instance. His arrays can also detect different types of organic solvents in industrial processes.

Although the sensors themselves are inexpensive to make, the technology required to link them together and process the information raises the cost. For this reason, Subramanian thinks the first applications of his sensors will be high-value ones, such as monitoring pharmaceutical packages. Test strips that diabetics use to detect glucose levels, for example, currently come with expiration dates; but these are imprecise, so strips often get thrown out that could still be used, he says. One of his sensor arrays in a package could detect the distinctive changes that occur when the strips go bad, reducing waste.

Applications such as these could be ready within a year, says Subramanian. But wide deployment in the environment will probably have to wait. “While environmental sensing is of great human importance,” he says, “it’s hard to get someone to pay for it.”

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