Challenges in the Design and Fabrication of a Lab-on-a-Chip Photoacoustic Gas Sensor

Abstract

The favorable downscaling behavior of photoacoustic spectroscopy has provoked in recent years a growing interest in the miniaturization of photoacoustic sensors. The individual components of the sensor, namely widely tunable quantum cascade lasers, low loss mid infrared (mid-IR) waveguides, and efficient microelectromechanical systems (MEMS) microphones are becoming available in complementary metal–oxide–semiconductor (CMOS) compatible technologies. This paves the way for the joint processes of miniaturization and full integration. Recently, a prototype microsensor has been designed by the means of a specifically designed coupled optical-acoustic model. This paper discusses the new, or more intense, challenges faced if downscaling is continued. The first limitation in miniaturization is physical: the light source modulation, which matches the increasing cell acoustic resonance frequency, must be kept much slower than the collisional relaxation process. Secondly, from the acoustic modeling point of view, one faces the limit of validity of the continuum hypothesis. Namely, at some point, velocity slip and temperature jump boundary conditions must be used, instead of the continuous boundary conditions, which are valid at the macro-scale. Finally, on the technological side, solutions exist to realize a complete lab-on-a-chip, even if it remains a demanding integration problem.

Downsizing and Silicon Integration of Photoacoustic Gas Cells

“…A 2-f detection scheme and a short integration time of 0.2 s led to a LOD 1σ ≈ 650 ppb, estimated from a linear fit of the measured data ( . A basic correction for the much larger volume (152 mm 3 vs. 40 mm 3 ) and much shorter integration time (0.2 s vs. 7 s) leads to a LOD with slightly reduced performances than that of the initial metal cell. Allan deviation plot provides information about the temporal stability of the experimental set-up as a whole [18,19].…”

“…Depending on the molecule of interest and the gas matrix, the relaxation time constant can vary on several orders of magnitude. Thus, in order to retain the possibility of detecting several different species with the same design of PA cell, it is advisable to fix an upper limit around 20 kHz [3].…”

“…As the photoacoustic signal is inversely proportional to the chamber volume [2], a straightforward way to improve the sensor resolution consists in reducing its size. This trend towards miniaturization which, when pushed to limits, faces some limitations [3], has been followed by several teams during the last decades [4]. We review hereunder the existing literature on miniaturized sensors from two different perspectives towards cost reduction: (i) additive manufacturing, nowadays limited to small-scale production, and (ii) silicon microfabrication in MEMS foundries, well adapted to high-volume production.…”

Multifidelity surrogate modeling based on radial basis functions

“…This pressure acoustic model is computationally efficient and accurate at the macro-scale but fails at the micro-scale (Glière et al 2014). In fact, various volume and surface dissipation processes, at work in the bulk of the propagation medium and close to the walls, cannot be neglected in miniaturized devices, where boundary layers occupy a non-negligible part of the overall cell volume.…”

Continuous Monitoring of Air Purification: A Study on Volatile Organic Compounds in a Gas Cell

“…TDLS and photoacoustic-based TDLS are characterized by high sensitivity and selectivity compared to FTIR spectroscopy, however, FTIR spectroscopy has a much larger spectral range which means the capability of measurement of a wide variety of different gases at the same time. Photoacoustic-based TDLS is characterized by independence of the sensitivity on gas path length allowing to use small gas cells of a few cm in length [23] and further miniaturization of the whole system using semiconductor technology [24]. Using Microelectromechanical systems (MEMS) technology, one could have a miniaturized gas sensor based on the FTIR principle, leading to a compact and cheap on-site and timely measurement [25].…”