Non-invasive testing is a key area of research in medicine, and in recent years, the detection of gases expelled from the body for disease diagnosis has gained considerable attention. Carbon dioxide (CO₂) in the skin, a major product of metabolism, reflects the body’s metabolic rate and the integrity and barrier function of the skin’s keratin layer. It can also serve as a non-invasive method to quickly and conveniently assess the health status of the body.

In this paper, skin CO₂ is used as the detection target, with quartz-enhanced photoacoustic spectroscopy (QEPAS) combined with first harmonic frequency locking. Initially, the R18 spectral line of CO₂, located at 4991.26 cm^-1, is selected by simulating the absorption spectral lines of CO₂ and interfering gases in the detection environment. A QEPAS system for skin CO₂ detection is then built based on a 2004 nm distributed feedback (DFB) laser, and the laser’s output frequency is locked at 4991.26 cm^-1 using the first harmonic signal. After frequency locking, the detection speed of the QEPAS system increases to 2.5 Hz, with the system’s linearity reaching 0.998 and a detection limit of 2.04×10^-6. The real-time detection of CO₂ metabolism in different parts of human skin is realized using this system.

By combining QEPAS with the first harmonic frequency locking, the first harmonic signal from the sample absorption cell is used as the input for the proportional-integral-derivative (PID) control system, locking the DFB laser’s output wavelength at 2004 nm to the CO₂ absorption line at 4991.26 cm^-1. This improves the system’s detection speed from 200 mHz to 2.5 Hz, enabling real-time monitoring of CO₂ volume fraction changes. Analysis of the system’s linearity and Allan variance shows a linearity of 0.998 and a detection limit of 2.04×10^-6. Compared to conventional QEPAS systems using non-frequency-locked wavelength scanning, the system’s detection sensitivity is unaffected while its detection speed is increased to 12.5 times that of traditional systems. Based on this system, real-time monitoring of skin CO₂ is conducted on four different parts of the human body: the left palm, left arm, left armpit, and left cheek. It is found that the CO₂ discharge rate from the left cheek is significantly higher than that of the other three parts. This demonstrates the feasibility of using the first harmonic frequency-locking technique to enhance the detection speed of QEPAS for trace gas detection.

The skin CO₂ gas sensor proposed in this paper offers advantages such as low cost, compact size, and high detection sensitivity, enabling convenient and rapid non-invasive detection of CO₂ released from human skin. This provides a simpler and more efficient alternative for the increasingly burdensome task of medical diagnostics. The sensor can be used for screening hypercapnia and hypocapnia, as well as for monitoring activity in transplanted skin. In this paper, the QEPAS technique is combined with the first harmonic frequency locking. The first harmonic signal from the absorption cell is used as the PID input signal to lock the output wavelength of the DFB laser at 2004 nm to the CO₂ absorption line at 4991.26 cm^-1, resulting in an improved system detection speed from 200 mHz to 2.5 Hz, and enabling real-time detection of CO₂ volume fraction changes. The system linearity is analyzed through Allan variance, yielding a linearity of 0.998 and a detection limit of 2.04×10^-6. Comparing the performance of this system with conventional QEPAS systems using non-frequency-locked wavelength scanning shows that although the detection speed is increased by 12.5 times, the detection sensitivity is still not affected. Using this system, real-time monitoring of skin CO₂ levels is conducted on four different parts of the human body: the left palm, left upper arm, left armpit, and left cheek. Notably, the CO₂ discharge rate from the left cheek is significantly higher than that of the other three parts, demonstrating the effectiveness of the first harmonic frequency-locking technique in improving the detection speed of the QEPAS trace gas detection system. The skin CO₂ gas sensor combines high detection sensitivity with low cost and compact size, facilitating quick and non-invasive CO₂ detection on the skin. This method offers an alternative for simplifying medical diagnostic tasks, with applications in hypercapnia and hypocapnia screening, as well as monitoring the activity of transplanted skin.