Moisture-resistant, stretchable NOx gas sensors based on laser-induced graphene for environmental monitoring and breath analysis

Abstract

The accurate, continuous analysis of healthcare-relevant gases such as nitrogen oxides (NO_x) in a humid environment remains elusive for low-cost, stretchable gas sensing devices. This study presents the design and demonstration of a moisture-resistant, stretchable NO_x gas sensor based on laser-induced graphene (LIG). Sandwiched between a soft elastomeric substrate and a moisture-resistant semipermeable encapsulant, the LIG sensing and electrode layer is first optimized by tuning laser processing parameters such as power, image density, and defocus distance. The gas sensor, using a needlelike LIG prepared with optimal laser processing parameters, exhibits a large response of 4.18‰ ppm^-1 to NO and 6.66‰ ppm^-1 to NO₂, an ultralow detection limit of 8.3 ppb to NO and 4.0 ppb to NO₂, fast response/recovery, and excellent selectivity. The design of a stretchable serpentine structure in the LIG electrode and strain isolation from the stiff island allows the gas sensor to be stretched by 30%. Combined with a moisture-resistant property against a relative humidity of 90%, the reported gas sensor has further been demonstrated to monitor the personal local environment during different times of the day and analyze human breath samples to classify patients with respiratory diseases from healthy volunteers. Moisture-resistant, stretchable NO_x gas sensors can expand the capability of wearable devices to detect biomarkers from humans and exposed environments for early disease diagnostics.

Keywords: Electronic properties and materials; Environmental, health and safety issues.

Fig. 1. Moisture-resistant, stretchable gas sensing systems based on laser-induced graphene (LIG) foams for environmental monitoring and patient breath analysis.

a Schematic illustrating NO_x (NO₂ or NO)-related air pollution and the use of NO_x gas as a biomarker for representative human diseases. b Exploded view of the LIG-based moisture-resistant, stretchable gas sensor to show its structural layout. c Optical image of a representative LIG-based gas sensor attached below the nose of a human subject, with a zoomed-in view of the sensor and the scanning electron microscope (SEM) image of needlelike LIG shown in the inset. d Images of the stretchable LIG-based gas sensor before and after varying deformations (i.e., stretching, twisting, and coiling onto the finger)

Fig. 2. Characterizations of the LIG prepared with different laser processing parameters.

a–d SEM images of the LIG prepared by laser powers of a 0.15 W, b 0.6 W, c 1.2 W, and d 1.8 W for a scanning speed of 2.54 mm/s and an image density of 500 PPI. SEM images of the LIG were prepared by the image density of e 750 PPI and f 1000 PPI for a laser power of 0.60 W and scanning speed of 2.54 mm/s. g Raman spectra of various LIG samples in a–d. h X-ray photoelectron spectroscopy (XPS) survey spectra of the LIG prepared by the optimal laser processing parameters (i.e., laser power of 0.6 W, image density of 500 PPI, and scanning speed of 2.54 mm/s)

Fig. 3. Effects of laser processing parameters on gas sensing performance.

Sensing performance of the LIG-based gas sensors prepared with different a powers and b image densities to 1 ppm NO gas, with their specific surface areas shown in c and d

Fig. 4. Gas sensing performance evaluation of the LIG-based gas sensor prepared with the optimal laser processing parameters.

a Typical response curve of the gas sensor to 1 ppm NO. b Dynamic response curves of the gas sensor to NO with concentrations increasing from 0.5 to 2.5 ppm. c Repeatability test of the gas sensor to 1 ppm NO for eight consecutive cycles. d Calibration curve with a linear fit obtained from the sensor response to NO from 200 to 1000 ppb. e Experimental demonstration of the sensor response to 20 ppb NO. f Selectivity test of the gas sensor to NO_x over other interfering gases

Fig. 5. Sensing mechanism and equivalent circuit diagram of the LIG-based gas sensor.

a Sensing mechanism of the chemiresistive LIG-based gas sensor for NO_x. b Equivalent circuit diagram of the gas sensor

Fig. 6. The moisture-resistant and stretchable performance of LIG-based gas sensor.

a Schematic illustration showing humidity-free gas sensing under wet conditions. Response of gas sensors in different relative humidity environments b without and c with a semipermeable PDMS membrane, with the water contact angle shown in the inset. d Thickness effect (10–20 μm) of the semipermeable membrane on the gas sensing response. e Response of the LIG-based gas sensor to 1 ppm NO upon stretching from 0 to 30%

Fig. 7. Demonstration of the LIG-based gas sensor for environment monitoring and breath analysis.

a Resistance variations of the LIG-based gas sensor to monitor outdoor air at different times of the day (i.e., morning, noon, and evening). b Comparison between the actual NO₂ concentrations in the environment (top) and the response from the gas sensor (bottom). Response of the gas sensor c without and d with the semipermeable membrane to human exhaled breath samples from patients with respiratory diseases and healthy volunteers