Atom-thin sensors convert breath and sweat into real-time health data

Researchers at Gauhati University have reviewed the development of wearable chemical sensors utilizing two-dimensional (2D) materials such as graphene derivatives, MXenes, and transition metal dichalcogenides (TMDs). These materials are pivotal in detecting volatile organic compounds (VOCs) and humidity in real time. The review, published in Frontiers in Sensors, highlights the unique properties of these atomically thin materials, including their large surface area and mechanical flexibility, which make them ideal for wearable devices that require prolonged skin contact. The study, led by Anurag Kashyap, Rajnandan Lahkar, and Hemen Kalita, addresses the challenge of sensing VOCs and humidity simultaneously, as these elements are often intertwined in real-world environments.

VOCs, such as ammonia and acetone, are not only industrial pollutants but also biomarkers detectable in exhaled breath, providing insights into conditions like chronic kidney disease and lipid metabolism. Humidity levels also affect health and the environment, influencing microbial growth and air quality. Wearable sensors capable of monitoring these variables continuously could revolutionize non-invasive diagnostics, currently reliant on clinical visits and complex instruments.

The review categorizes wearable sensors by their substrates, including polymers, textiles, and paper. Each substrate offers distinct advantages and limitations. Polymers like polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) are elastic and skin-friendly but raise disposal concerns. Textiles offer comfort and adsorption properties but are underexplored for VOC and humidity detection. Paper is cost-effective and biodegradable but lacks mechanical durability. Despite these challenges, most 2D-material sensors are currently based on polymers or paper, highlighting a potential area for further research.

Chemiresistive sensors dominate the wearable sensor landscape due to their simplicity and low power requirements. These sensors operate by adsorbing VOC or water molecules, which alters their electrical resistance. The review discusses how surface chemistry and heterostructure formation can enhance sensor performance. For instance, stacking 2D materials like WS2 on MoS2 creates an electric field that amplifies resistance changes. Impedance-based readouts offer an alternative by separating bulk and interfacial contributions, reducing humidity interference.

Practical applications of these sensors are already emerging. A graphene/PANI hybrid sensor on polypropylene masks demonstrated significant sensitivity to ammonia, while an organic field-effect transistor sensor achieved high sensitivity and stability in humid conditions. MXenes and TMDs also show promise, with sensors capable of detecting ethanol and nitrogen dioxide efficiently. Metal-organic frameworks (MOFs) further enhance selectivity when combined with 2D materials, as demonstrated by a nickel-MOF-on-graphene sensor that accurately identified lung cancer biomarkers.

The review outlines future directions for the field, emphasizing the need for improved selectivity, energy harvesting for self-powered devices, and biocompatible encapsulation. Scalable fabrication techniques and standardized testing protocols are crucial for advancing these technologies from the laboratory to the market. The authors envision a future where biodegradable substrates and AI-assisted data fusion enable continuous, personalized health monitoring through wearable sensors.