The precise, continuous monitoring of pollutants such as nitrogen oxides (NO_x) in high humidity is a tough challenge for low-cost and stretchable gas sensors.

A recent article published in Microsystems & Nanoengineering describes the design and implementation of a novel, moisture-resistant, and wearable NOx gas sensor based on laser-induced graphene (LIG) that has proven successful in monitoring the environment and classifying individuals with breathing problems.

Importance of Nitrogen Oxides (NOx) Detection

Nitrogen oxides (collectively referred to as NO_x) produced by combustion processes and petroleum refineries are significant air pollutants that cause bronchitis, asthma, and heart-aggravating disorders.

Nitric oxide (NO), a key biomarker for pulmonary inflammation, is of great relevance for the non-invasive detection and treatment of respiratory disorders such as lung cancer and ventilator-associated pneumonitis. This is because the level of nitric oxide in exhaled breath of asthma patients surpasses hundreds of parts per billion (ppb), but this amount is less than a few tens of ppb in healthy individuals.

Consequently, it is of the utmost significance to create new, inexpensive, and dependable gas sensors for continuously and precisely detecting nitrogen oxide (NO_x) in human breath.

Wearable Graphene-based Gas Sensors for Detection of NOx

Several technologies using nanoparticles such as metal oxides, graphene, carbon nanotubes, and conductive polymers have been developed to detect NO_x in human breath.

In contrast to the electrolytic cells, field-effect semiconductors and other conventional gas sensors, wearable NO_x gas sensors based on graphene exhibit low noise and good mechanical robustness.

Wearable electronic devices can gather electrical, biochemical, thermal, physical, and biological information for comprehensive health management. The growing interest in personalized air quality assessment and breath testing has also increased the need for wearable graphene-based gas sensors that can detect different harmful chemicals, such as nitrogen oxides (NO_x), precisely and continuously.

Graphene-based gas sensors with unmodified surfaces show low accuracy due to the absence of active sites. To tackle this issue, the newly discovered 3D laser-induced graphene (LIG) can be utilized because it provides a large number of active sites on the surface for gas-solid interactions.

Relative Humidity: A Major Limitation in Accurate Gas Sensing

As water particles populate the active surface functional sites of sensing nanomaterials, the relative humidity (RH) significantly affects the absorption and equilibrium mechanisms of the target gas, resulting in substantial response changes in breath samples with an RH between 50 and 95 percent.

The impact of relative humidity on gas sensing can be minimized by utilizing coated integrated heating components, hydrophobic self-assembled monolayers (SAMs), or electronic nose algorithms. However, these technologies often increase the sophistication and expense of gas sensor production.

Therefore, it is crucial to establish a simple method for designing and fabricating moisture-resistant NOx gas sensors with a large response rate, quick response/recovery, and a low limit of detection (LOD) for monitoring local air pollution and analyzing breath samples for disease diagnoses.

A Novel Moisture-Resistant LIG-based NOx Gas Sensor

In this study, the researchers created a novel, flexible, and moisture-resistant LIG-based NO_x gas sensor by sandwiching the LIG sensing area between a semi-permeable polydimethylsiloxane (PDMS) layer and a flexible elastomeric platform.

The flexible, extremely sensitive, and selective LIG-based gas sensor is composed of a straight LIG sensing area and a serpentine electrode on a soft elastomeric platform. The LIG sensing region has a much narrower width (150 micro-meter) than the electrodes to produce substantially stronger resistance and concentrated Joule heating in the sensing zone,

The rapid and low-cost manufacturing method is also scalable, promising rapid large-scale production for commercial applications in the future.

Key Developments of the Research

At room temperature, the as-prepared gas sensor displays a high response rate, rapid response/recovery, and an ultralow limit of detection (LOD) for various nitrogen oxides (NO_x). By adjusting laser processing parameters such as laser intensity, image frequency, and defocus distance, the laser direct writing method can produce LIG sensing zones with varying morphologies.

This unique water-resistant LIG-based gas sensor can detect the outside air quality at various times of the day by combining a high stretchability of 30 percent and a moisture-resistant capability against a relative humidity of 90 percent. In addition, it can promptly evaluate clinical breath samples to properly distinguish between patients with respiratory illnesses and healthy human beings.