The World Health Organization has defined cardiovascular diseases as the number one cause of death in the world [1]. The diagnosis of these diseases can be invasive for the patient. With the introduction of new technologies in the medical field, scientists and doctors are working together to find new and less invasive ways to ease medical procedures. For many years, scientists have shed light on several gases present in the exhaled breath: water vapor, nitrogen, oxygen, carbon monoxide (CO)... Some of them are biomarkers and their investigation can lead to the diagnosis of several diseases. We present a Quartz enhanced photoacoustic (QEPAS) [2] sensor based on infrared lasers, dedicated to CO, nitric oxide (NO), and acetone monitoring. CO sensing is performed with a 4.7 um quantum cascade laser. This sensor has proved its sensibility and selectivity with a limit of detection of 20 ppbv in 1s. Therefore, further measurements were also performed in situ in the hospital to confront the sensor to a medical sensor. We have demonstrated the influence of the breath hold, characterized different respiratory compartments and discriminated smokers and non-smokers volunteers [3]. These first measures made on humans have brought out some physiological points that need to be taken in account. The QEPAS signal depends on the resonance frequency (f0) of the quartz tuning fork (QTF). The humidity naturally present in breath causes a shift of f0. Some improvements have been proposed to track f0 and the QTF Q-factor to stabilize the measurement [4].
Quartz-enhanced photoacoustic spectroscopy (QEPAS) [1] is one of the most efficient ways to obtain sensitive, selective, robust gas sensors. The main drawback of QEPAS comes from usage of quartz tuning fork (QTF) as a mechanical transducer. QTF is not specifically design for photoacoustic gas sensing and its further integration is limited. As a solution we propose a silicone resonant MEMS based on capacitive transduction mechanism. This sensor, specifically designed for acoustic sensing purposes, can be an efficient transducer for sound wave detection able to advantageously replace a QTF. Capacitive transduction allows reaching high sensitivity of the sensor while choice of silicon is favorable in design flexibility, fabrication maturity, stability and further integration with CMOS electronics. We have developed and fabricated various resonator designs on silicon. Specific designs were created to sensor voltage output using an analytic model developed by our group [2]. Photoacoustic measurement was performed on calibrated mixtures of methane using commercial Eblana distributed feedback laser laser emitting at 1.63 μm. We achieved a reproducible limit of detection on methane: 1000ppmv in 5s for 2f detection and 700ppm in 5s for 1f detection (figure 1) (resonator resonance frequency 22.65 kHz and Q-factor of 250). Then, we compared the experimental results with standard QTF in off-beam configuration for which the limit of detection: 30ppmv in 5s for 2f detection and 25 ppm in 5s for 1f detection. Thus, the difference of detection limit between QTF and MEMS amounts factor 28 for 1f detection and 33 for 2f detection. [1] Kosterev, A. A., Bakhirkin, Y. A., Curl, R. F., & Tittel, F. K. (2002). Quartz-enhanced photoacoustic spectroscopy. Optics letters, 27(21), 1902-1904. [2] Trzpil, Wioletta, et al. "Analytic Optimization of Cantilevers for Photoacoustic Gas Sensor with Capacitive Transduction." Sensors 21.4 (2021): 1489.
Gas sensing find tremendous applications in various fields like medicine, air quality, food processing or security and defence. The main challenge in industry is to create an integrated and compact sensor while maintaining its performance and power consumption. Photoacoustic spectroscopy (PAS) gains particular interest in this field due to its excellent selectivity while maintaining compactness. In tunable laser diode absorption spectroscopy (TDLS) the signal is proportional to optical path. Sensitivity in photoacoustic spectroscopy is proportional to the power of the laser, which allows to keep a good sensitivity even with small gas cells. The use of mechanical resonator with high quality factor allows improving the signal-to-noise ratio and avoid the use of an acoustic chamber. Micro-electro mechanical systems (MEMS) fabricated in silicon technology remain a reasonable choice to realize a compact and integrated sensor, including laser source and electronics. We propose a capacitive transduction method, which can be easily integrated, compact and highly sensitive. Due to the multi-physics problem, time and financial contains, a theoretical model seems to be a first step towards sensor performance improvement. We propose an analytical model for a new concept of photoacoustic gas sensing using capacitive transduction mechanism. The model was reinforced with computational methods implemented in Python programming environment. The study was carried out using silicon cantilever as a model, which brings an opportunity to obtain an analytical solution for all physical parameters. The goal of this research stands maximization of electrical signal output and signal-to-noise (SNR) ratio. Conducted study provides a solution to retrieve a cantilever dimensions and frequency for integrated compact gas sensor. Beyond optimization, the model provides a comprehensive tool to understand mechanisms of sensor working principles and therefore stands as a tool allowing a mechanical resonator to be developed with a more complex geometry and/or different transduction mechanism.
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