Jan 31 2020
A study by scientists from Oregon State University has reached a step closer to creating an electronic nose for detecting safety threats, monitoring air quality, and diagnosing diseases by quantifying gases in the breath of a patient.
Depiction of a gas sensor array composed of microscale balances coated with thin films of nanoporous materials called metal-organic frameworks. Image Credit: Arni Sturluson, Melanie Huynh, OSU College of Engineering.
Headed by Cory Simon, a research assistant professor of chemical engineering in the OSU College of Engineering, in collaboration with Chih-Hung Chang, a chemical engineering professor, a study reported recently focused on materials called metal-organic frameworks (MOFs).
The objective of the study was a crucial yet less-explored hurdle in the use of MOFs as gas sensors: what is the way to identify, out of the billions of probable MOFs, the suitable ones for developing the ideal electronic nose?
MOFs includes pores that are nanosized and selectively adsorb gases, analogous to a sponge. Thanks to their tunability, they are suitable for use in sensor arrays, thus allowing engineers to employ a diverse set of materials that enables an array of MOF-based sensors to offer detailed information.
Based on the components that constitute a gas, various amounts of the gas will adsorb in each MOF. This implies that the composition of a gas can be known by quantifying the adsorbed gas in the MOF array with the help of micro-scale balances.
The problem faced is that all gases are adsorbed by all MOFs—not to the same degree, but yet the lack of perfect selectivity prevents an engineer from just saying, “let’s just dedicate this MOF to carbon dioxide, that one to sulfur dioxide, and another one to nitrogen dioxide.”
Curating MOFs for gas sensor arrays is not that simple because each MOF in the array will appreciably adsorb all three of those gases.
Cory Simon, Research Assistant Professor of Chemical Engineering, College of Engineering, Oregon State University
This challenge is overcome by human noses by depending on nearly 400 different types of olfactory receptors. Quite similar to the MOFs, many distinct odors activate each olfactory receptor, and many different receptors are activated by each odor. The brain analyzes the response pattern, thus enabling people to differentiate between several different odors.
In our research, we created a mathematical framework that allows us, based on the adsorption properties of MOFs, to decide which combination of MOFs is optimal for a gas sensor array.
Cory Simon, Research Assistant Professor of Chemical Engineering, College of Engineering, Oregon State University
Simon added, “There will inevitably be some small errors in the measurements of the mass of adsorbed gas, and those errors will corrupt the prediction of the gas composition based on the sensor array response. Our model assesses how well a given combination of MOFs will prevent those small errors from corrupting the estimate of the gas composition.”
According to Simon, despite the fact that the study mainly involved mathematical modeling, experimental adsorption data in real MOFs was used by the researchers as input. He added that Chang is an experimentalist “who we are working with to make a real-life electronic nose to detect air pollutants.”
“We are currently seeking external funding together to bring this novel concept into physical realization,” stated Simon.
Because of this paper, we now have a rational method to computationally design the sensory array, which encompasses simulating gas adsorption in the MOFs with molecular models and simulations to predict their adsorption properties, then using our mathematical method to screen the various combinations of MOFs for the most accurate sensor array.
Cory Simon, Research Assistant Professor of Chemical Engineering, College of Engineering, Oregon State University
This means rather than using an experimental trial-and-error method to identify the MOFs to be used in a sensor array, engineers can utilize computational power to curate the best array of MOFs for an electronic nose.
The diagnosis of diseases is another fascinating application of such a nose. The volatile organic compounds emitted by humans, for example, through their breath, contain biomarkers for various diseases. Research works have demonstrated that dogs—with twice the number of distinct olfactory receptors as humans—have the ability to detect diseases with their nose.
Although dogs’ noses are magnificent, they are not so practical for extensive diagnostic use like a thoroughly crafted and produced sensor array.
The outcomes of the computational MOF study were reported in ACS Applied Materials & Interfaces.
The first author of the study was Arni Sturluson a chemical engineering PhD student at OSU. The collaborating authors were PhD student Yujing Zhang and undergraduates Rachel Sousa, Melanie Huynh, Caleb Laird, Arthur H.P. York, and Carson Silsby.