Sniffing Out the Foundational Science of Sensors

Estimated reading time: 9 minutes

The human nose can distinguish among a trillion different combinations of smells. Even so, there are plenty of gases that our noses can't detect at the level of sensitivity we need. That's where gaseous sensors come in. While some of the first sensors were animals – like canaries in coal mines – we've since replaced them with technologies that can detect miniscule amounts of chemicals in the air.

Image caption: This sensor can detect methane at much lower concentrations than current ones. It relies on nanotechnology developed at the Center for Nanoscale Materials, an Office of Science user facility.

Just like our own noses, gaseous sensors are essential for safety and comfort. In factories, gaseous sensors can alert managers to chemical leaks or processes running incorrectly. Outside, they measure pollutants, helping cities monitor air quality. In homes, they keep family members safe. Building managers use measurements from humidity and temperature sensors to maximize energy efficiency.

These sensors wouldn't exist without a fundamental understanding of chemistry and physics. This basic knowledge helps scientists understand how and why sensing materials interact with gaseous chemicals. Many cutting-edge materials have promise for use in sensors, if only scientists can learn how to better produce and control them.

"Sensors are where materials research meets environmental detection," said Pete Beckman, a researcher at Department of Energy's Argonne National Laboratory (ANL).

To set the groundwork for innovation, the DOE Office of Science funds projects and user facilities that support sensors research.

Creating the Materials for Sensing

Like noses, sensors rely on a combination of components to detect and make sense of gases or chemicals in the air. In humans, molecules float up into your nose and bind to special neurons. Neurons then pass the message up to the brain. In sensors, the material inside the sensor acts as a neuron. When that material interacts with a chemical in the air, it may emit light, change its ability to conduct electricity, or shift shape. The materials and electronics around the sensing material communicate that message to the sensor's "brain," whether that brain is a computer or a warning signal like a siren.

Developing sensors' nervous systems and brains is a job for applied science. Fundamental research like the work at the Office of Science's laboratories sets the foundation for that applied science. In particular, this research is expanding scientists' understanding of the materials themselves and how to produce them.

Three types of cutting-edge materials offer huge potential for use in sensors: nanoparticles, two-dimensional (2D) materials, and metal-organic frameworks (MOFs). Nanoparticles are miniscule particles that are bigger than atoms, but act fundamentally differently from larger particles of the same substance. 2D materials, like graphene, form sheets only a single atom thick. MOFs are compounds made of metal ions linked together by carbon-based connectors.

All of these materials have humongous surface areas compared to their overall sizes. Because lots of gas molecules can interact with their surfaces, they can be sensitive to tiny amounts of chemicals. In addition, scientists can craft all of these materials into a variety of structures. That customization could allow researchers to create specialty materials to detect a particular chemical.

Zinc Sulfide Nanoparticles

The key to building a better sensor may lie in making its sensing material out of nanoparticles. Unfortunately, it's challenging to manufacture some of the most promising of those nanoparticles. Sensors for hydrogen and other gases already use the material zinc sulfide. Producing zinc sulfide in nanoparticle form could make it cheaper and more effective. But the current process for producing zinc sulfide nanoparticles involves very high temperatures, pressures, and toxic chemicals.

Scientists at DOE's Oak Ridge National Laboratory (ORNL) investigated a cheaper, more efficient nanoparticle production process. Researchers supported by both DOE's Advanced Manufacturing Office and Office of Science found that microbes may offer an alternative path forwardExternal link.

Not just any bacteria will do. Scientists used Thermoanaerobacter, a bacteria that normally lives in extremely hot places with no oxygen. After adding a cheap sugar and chemicals that included zinc and sulfur, the bacteria produced about three-quarters of a pound of zinc sulfide nanoparticles. The process was 90 percent cheaper than current methods.

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