Team Builds Low-Cost, Low-Energy Carbon Dioxide, Humidity Sensor

A research team at The University of Texas at Dallas has created a first-of-its-kind sensor for real-time measurements of carbon dioxide and relative humidity — using a technique conceived while washing dishes.

“As environmental concerns continue to mount, we’re seeking new ways to monitor atmospheric conditions,” said Dr. Shalini Prasad, Cecil H. and Ida Green Professor in Systems Biology Science and the interim department head of bioengineering in the Erik Jonsson School of Engineering and Computer Science. “With this prototype, we’ve created something appropriate for usage in automobile and smart-phone manufacturing, as well as in the monitoring of energy-efficient buildings — all enabled through the internet of things.”

Ashlesha Bhide, a UT Dallas bioengineering PhD student and research assistant working with Prasad, was the lead author of a recent paper in the Journal of Solid State Science and Technology, and another in ECS Transactions, outlining how room temperature ionic liquid (RTIL) could be a novel, electrochemical-sensing element.

RTILs, which also are featured in a diabetes-tracking sensor project from Prasad’s lab, are gels made from molten salt that allows a sensor to perform at a high level for a long time due to the substance’s high thermal stability and low volatility. The idea of repurposing this existing technology for her research came to Prasad during a simple chore.

“RTIL technology has been around since the 1960s — in chemistry applications, for lubricants and batteries,” she said. “One evening, I was washing dishes, having used a lot of cooking oil. This oil had encapsulated some food particles in a pan. The food stayed trapped in this bubble even as I applied more and more water pressure. I eventually had to poke it with a fork.

“That got my mind trying to mimic this behavior in something I could use. And that, in essence, is how an RTIL sensor grabs and suspends the sample it collects, keeping it unchanged under pressure.”

More Durable, Less Expensive

The UT Dallas team tracked the effects of carbon dioxide concentrations and relative humidity levels across temperatures on three RTILs, and showed that the liquids present a viable, low-energy solution.

“The RTIL-coating element has been engineered specifically for CO₂ measurements and can withstand temperatures from minus 45 to 400 Celsius,” Prasad said. “We’ve also made it amenable for the kind of printing that existing semiconductor processes use, which has been a big roadblock for integration of new technologies; the semiconductor fabrication process is resistant to the introduction of new steps.”

The project was a collaboration with Texas Instruments (TI), a leading manufacturer of semiconductor computer chips that underpin many electronics products — and an indelible part of UT Dallas’ history, as TI’s three founders are also the University’s founders.

“These next-generation, low-power CO₂ and humidity-monitoring sensors have an increased potential field of use, allowing TI to reach larger market segments,” Prasad said.

The prototype is more compact and durable than current technology that often costs hundreds of times more than her mass-produced RTIL sensor will.

“Any sensor for this purpose would have to be highly sensitive and stable while using little power,” Prasad said. “These requirements make RTIL an ideal material. Our sensor is three times faster, has a wider dynamic range and is significantly more sensitive than the current state-of-the-art sensor.”

Broad Applications

Prasad said that carbon dioxide sensors are being used for pollution monitoring via cellphone in the European Union, as well as real-time monitoring of engine health and emission-standard compliance in cars worldwide. This creates the need for the new monitor.

Dr. Srikanth Krishnan, reliability manager and distinguished technical staff member at TI, elaborated on the collaboration and key findings.

“Dr. Prasad’s group has shown its tool has excellent sensitivity and stability at temperatures up to 200 degrees Celsius, making RTIL a good candidate for broad applications,” Krishnan said. “The prototype developed shows the ease of developing a complete sensing module with high functionality.”

Among the Texas Instruments collaborators was Dr. Rujuta Munje, a former member of Prasad’s lab who earned her PhD from UT Dallas in 2016 and was hired into the TI sensors group. Munje, who performed foundational work on this concept while at UT Dallas, described some of the benefits such a tool would have.

“From air quality in large commercial and industrial buildings to quality and safety in food and agricultural settings, CO₂ sensors play many important roles already,” Munje said. “Immediate, accurate data will help companies conform to new environmental guidelines and collect sufficient information to adjust in real time.”

Prasad said that humidity monitoring is similarly important for health and productivity reasons.

“This is critical in both environmental and industrial spaces, to keep people healthy and to keep machines running efficiently,” she said. “From health care facilities to data centers, water vapor levels can have a variety of negative effects.”

“This technology can allow hospitals or homes to act upon changes in air quality,” Munje said. “This would represent a huge step in functionality over the products that do this today.”

According to Prasad, the next step for these specific sensor prototypes is field testing. For RTILs, her group is exploring the potential of sensing and isolating specific gases.

“Detection and sequestration of reactive and nonreactive gaseous species could have applications ranging from environmental safety to precision testing of automotive quality,” Prasad said.

This work was funded through Texas Instruments and the Semiconductor Research Corp., an industry consortium of which TI is a member.