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Gas sensors get a big boost in sensitivity

By Paul Grad |

It has been a challenge to find materials that can reliably detect volatile organic compounds (VOCs) and toxic gases in air at the parts-per-billion (ppb) level, or that can be used for therapeutic diagnosis from breath analysis. That’s because two requirements must be simultaneously met to obtain very high sensitivity in typical resistive sensors: low electrical noise, due to high conductivity; and high signal, due to strong and abundant analyte-adsorption sites. These two requirements are, however, usually in a trade-off relationship.

Now researchers from Drexel University (Philadelphia, Pa.; www.drexel.edu) and the Korea Advanced Institute of Science and Technology (KAIST; Daejeon; www.kaist.ac.kr) have demonstrated that a 2-D, titanium-based MXene, (Ti3C2Tx, where T = O, F, OH and so on) can be used as a highly selective detector of gases.

MXenes are a large family of 2-D transition metal carbides or nitrides, where representative MXenes such as Ti3C2(OH)2 possess metallic conductivity, while the outer surface is fully covered with functional groups. Therefore, metallic conductivity and abundant surface functionalities may coexist without mutual interference. Such a combination makes them highly attractive for gas sensors with a high signal-to-noise ratio. This indicates the relative gas signal intensity over noise intensity, as the high coverage of functional groups allows strong binding with analytes, while the high metallic conductivity intrinsically leads to low noise.

According to the team, the signal-to-noise ratio of the Ti3C2Tx sensors was up to two orders of magnitude higher compared with that of all other 2-D materials. Sensors used today have a signal-to-noise ratio of 3–10, while MXene’s is between 170 and 350.

Although only one type of MXene was studied, it well exceeded the performance of conventional materials. The researchers expect that their study will open the door for a large family of Mxenes that could potentially be employed as highly sensitive sensors.

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