Smoke crawling over smoke detector.

[Image above] Gas sensors allow us to engineer devices, like this smoke detector, to signal the presence of particular compounds. Credit: Drexel University

Have you ever wondered how the world would smell if you were a dog?

Dogs have incredible noses—their sniffers are tens of thousands of times more keen that our own (by average estimates—some put that figure at closer to 100,000 times better than our own, but of course that would depend upon the dog, and person).

Dogs have such sensitive snouts that they can detect some compounds at a parts per trillion level. That’s why dogs are excellent at rescue operations, hunting, detecting drugs and explosives, and even sniffing out human diseases.

To that last point, because human bodies are metabolic chemical factories, dogs can sniff out the volatile organic compounds (VOCs) expelled from our bodies—and unique chemical signatures of these VOCs or the presence of particular compounds can provide important clues about the status of our health.

While we humans don’t have as good of sniffers as our canine companions, what we lack in olfactory abilities we make up for with our big brains—which allow us to engineer clever solutions that mimic what nature does so efficiently.

So humans have engineered materials into gas sensors that can sniff out harmful gases, detect toxic fumes, monitor pollution, and even check up on diseases.

Although we have gas sensors that can efficiently detect carbon monoxide in our homes, for example, gas sensors to detect and monitor diseases still need a little work—we need better materials that come with not only increased sensitivity, but also reduced noise—to be able to accurately detect, diagnose, and monitor our health.

For example, to be able to detect diabetes, gas sensors must be able to accurately detect acetone compounds on the breath at just 300–1,800 parts per billion. Or to detect peptic ulcers, gas sensors must detect ammonia molecules at 50–200 parts per billion on the breath. That’s a sensitivity that may come easy for my dog Murphy, but it’s no simple task to materials engineers.

My dog Murphy—who has quite a keen nose for peanut butter in particular.

In the case of gas sensors, the goal is low electrical noise and high sensitivity, which usually come at a trade-off within a material.

However, theoretical work has predicted that a newer class of conductive materials called MXenes could offer both, which would make them excellent gas sensors—and new experimental results now confirm this is the case.

MXenes are a family of 2-D materials composed of transition metal carbides and nitrides. First discovered at Drexel University (Philadelphia, Pa.) in 2011, the family of conductive materials have interesting and useful properties, yet can easily be manufactured, indicating they have promising commercial potential.

MXenes have been investigated for their ability to fabricate electromagnetic shields, promise faster charging batteries, offer energy storage solutions, and enable new methods of water purification, for instance. And now, add superior gas sensing ability to that list, too.

A group of researchers from Drexel University and KAIST in South Korea has shown that titanium carbide MXene thin films have superior gas sensing ability over existing gas sensor materials, making them particularly suitable for enabling the next generation of medical diagnostic sensor technologies.

The researchers fabricated Ti3C2Tx MXene thin films and tested their ability to sense various gases, including acetone, ethanol, ammonia, propanol, nitrogen dioxide, sulfur dioxide, and carbon dioxide.

Their results, reported in ACS Nano, show that MXene films can accurately and acutely detect the presence of each gas. Gas sensors detect presence of a gas by measuring a change in electrical conductivity in the material when the gas is present. The MXene sensors had the best response for ethanol, and higher selectivity for gases that hydrogen bond over acidic gases.

“MXene is one of the most sensitive gas sensors ever reported. This research is significant because it expands the range for detection of common gases allowing us to detect very low concentrations that we were not able to detect before,” Yury Gogotsi, Distinguished University and Bach Professor in Drexel’s College of Engineering and a lead author of the study, says in a Drexel news release. “The high sensitivity of the device may be used for detecting toxic gases or pollutants found in our environment.”

But not only can the materials sense a range of different gases, but they can do so very sensitively, indicating superior potential for medical gas sensors.

“MXene can detect gases in the 50–100 parts per billion ranges, which is below the concentration necessary for current sensors to detect diabetes and a number of other health conditions,” Gogotsi adds in the release.

With the help of density functional theory calculations, the scientists say that the MXene sensors are so efficient because of excellent conductivity of the metal core channels in the thin films and strong surface adsorption energy, likely due to the presence of hydroxyl groups.

And although the scientists only report the results for titanium carbide MXene materials so far, there are many other MXenes that may have sensor potential as well. Plus, the authors add in the paper, sensor selectivity could be further controlled via surface engineering, such as ligand functionalization or defect engineering.

“The next step to advance this research will be to develop sensor sensitivity to different types of gases and improve the detection selectivity between different gases,” Gogotsi says in the release. “We can also imagine personal sensors that will be in our smart phones or fitness trackers, monitoring body functions and the environment while we work, sleep or exercise, accessible with a tap of a finger. Improving the detection sensitivity with new materials is the first step toward making these devices a reality.”

The paper, published in ACS Nano, is “Metallic Ti3C2Tx MXene gas sensors with ultrahigh signal-to-noise ratio” (DOI: 10.1021/acsnano.7b07460).

Plus, MXenes aren’t only getting attention recently as gas sensors—Gogotsi also partnered with researchers from Missouri S&T to publish another paper recently about the potential of MXene materials as triboelectric nanogenerators (TENG) that can harvest wasted mechanical energy. Such MXene TENGs could enable self-powering devices that have the ability to harvest energy from muscle contractions, such as walking or chewing.

That paper, published in Nano Energy, is “Metallic MXenes: A new family of materials for flexible triboelectric nanogenerators” (DOI: 10.1016/j.nanoen.2017.11.044).

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