In light of global warming and its ever-increasing effects on climate change, stringent regulations have been put in place to minimize greenhouse gas emissions, particularly CO2. In addition, local pollutants such as volatile organic compounds (VOCs) and nitric oxides (NOx) negatively impact the environment [1, 2]. Such emissions are mainly produced from various anthropogenic sources such as industrial activity and transportation [3]. VOCs play a critical role in atmospheric reactions that lead to the formation of secondary pollutants [1]. VOCs can be toxic even at trace levels, thus monitoring their emission and abundance in workplaces and in the atmosphere is of vital importance. For instance, the 8-hour time-weighted average exposure limit of some VOCs can be as low as a few ppb [4]. Therefore, extensive efforts have been directed towards developing gas sensors capable of measuring such pollutants with high sensitivity and selectivity.
Multiple techniques and devices exist for monitoring pollutants in ambient air. Examples of such devices include Fourier-transform infrared spectrometers (FTIR), photoionization detectors (PIDs), amperometric sensors, and gas chromatography (GC). Most of these devices are bulky, require sampling, long measurement time, and operation by trained personnel [5]. Furthermore, their performance can be compromised by multiple environmental factors such as wind velocity, humidity, and temperature [5, 6]. Laser-based sensors are a better alternative for air quality monitoring on multiple fronts. These are portable, non-intrusive, have a high spectral, spatial, and temporal resolution, and are capable of measuring multiple species at once. Laser-based sensors can thus overcome many of the limitations associated with other speciation techniques [7]. Given these advantages, laser-based sensors have garnered attention from researchers and have been proven to be a useful tool for quantitative analysis of gaseous species in numerous fields, such as chemical analysis [8], monitoring of agricultural emissions [9], control of industrial processes [10], breath analysis [11], pollution monitoring [12–14], and combustion diagnostics [15–18].
Infrared laser absorption spectroscopy (IR-LAS) exploits vibrational and rotational energy modes of molecules and can thus provide species-specific detection in a multitude of applications. In particular, the mid-infrared (MIR) region contains the fundamental vibrational bands of most of the atmospheric species and VOCs (e.g., CO2, H2O, hydrocarbons, etc.) [19, 20]; the absorption coefficients of these species are orders of magnitude stronger in this region than in the near-infrared region [21]. A specifically interesting region in the MIR is the molecular fingerprint region (6.7–20 \({\mu }\)m) which includes strong bending vibrational modes of many air pollutants and the C-halogen stretching modes of most halocarbons [22]. Probing molecules in this spectral region could benefit from the distinct features of that region, which come from the highly intense and unique absorption coefficients of the CH bending modes of many pollutants of interest. For instance, BTEX (benzene, toluene, ethylbenzene, and xylene isomers) show overlapping absorption features over most of the IR range, which necessitates the use of advanced post-processing techniques in order to selectively measure BTEX species [23–25]. However, standard scanned-wavelength absorption spectroscopy would be sufficient for selective detection due to the isolated, distinct features of BTEX in the molecular fingerprint region.
Despite recent advances in laser technology (e.g., telecommunications lasers, inter-band cascade lasers, quantum cascade lasers (QCLs), and frequency combs) access to the long-wavelength MIR region (> 13 \({\mu }\)m) has been limited. This, in turn, hindered the development of laser-based sensors in that highly interesting region [22]. This limitation, combined with the aforementioned advantages of this spectral region, motivated researchers to use nonlinear conversion processes, such as difference-frequency generation (DFG) and optical parametric oscillation (OPO), to access such deep wavelengths in the IR region [26, 27]. For instance, DFG has been used to probe benzene by accessing its \(\nu\)4 band (Herzberg’s numbering) near 14.84 \({\mu }\)m [28, 29] and HCN by accessing its \(\nu\)2 band near 14 \({\mu }\)m [30]. However, such non-linear conversion techniques are complicated and involve bulky setups, which defies some of the main benefits of laser-based sensors.
Recently, long-wavelength distributed feedback (DFB) QCLs, fabricated via molecular beam epitaxy from the InAs/AlSb material family at Montpellier University, have been commercialized by mirSense (uniMir lasers). These are the first and only semiconductor lasers that operate in the continuous wave (cw) mode at room temperature in the long wavelength mid-infrared region (10–17 \({\mu }\)m) [31]. This technology opens a new horizon in laser-based sensing in the long-wavelength MIR region. Karhu et al. [32] utilized the newly developed DFB-QCL to demonstrate an ultra-sensitive benzene sensor that probed its \(\nu\)4 band near 14.84 \({\mu }\)m by making use of cantilever-enhanced photoacoustic spectroscopy (CEPAS). The achieved detection limit of the CEPAS sensor was 450 ppt at a long averaging time of 200 minutes. Recently, Ayache et al. used the same QCL technology to develop their own benzene sensor that probed the same band (\(\nu\)4) using quartz-enhanced photoacoustic spectroscopy (QEPAS) [33]; they successfully achieved a detection limit of 4 ppb at 2 minutes of averaging time. Shortly after, Karhu and Hieta improved their CEPAS sensor [32] by adding an adsorption enrichment stage, where benzene was collected on a sorbent and then detected from the enriched samples using photoacoustic spectroscopy [34]. This enrichment stage improved the detection limit and averaging time of their sensor to 150 ppt in 30 minutes (vs. 450 ppt in 200 minutes in their previous study [32]).
While photoacoustic spectroscopy (PAS) has several advantages such as high sensitivity and small sample volume, it is more complex and less robust than direct, single-pass, tunable diode laser absorption spectroscopy (TDLAS). PAS is typically used in extractive sampling applications since a separate photoacoustic gas cell is required. Furthermore, photoacoustic detection elements are typically impacted when operating in harsh environments, e.g., by temperature variations, mechanical vibration, external acoustic noise, and humidity [7]. Indeed, as water-vapor affects the relaxation time of detected molecules, the previously discussed PAS-based benzene sensors [32–34] were found to be heavily affected by the relative humidity. Thus, it was concluded that it might be necessary to measure the relative humidity of the measured sample and apply a correction to the measured benzene concentration accordingly [32, 33] [34]. Finally, a recent review by Fathy et al. [35] concluded that direct absorption spectroscopy outperforms photoacoustic spectroscopy in terms of sensitivity at such long IR wavelengths for low-to-medium-power lasers (< 1 W, which is the case for almost all cw lasers in the IR region).
Herein, we report the development of a multispecies laser-based sensor for the detection of benzene (C6H6), acetylene (C2H2), and carbon dioxide (CO2) near 14.84 \({\mu }\)m. The sensor is based on scanned-wavelength direct absorption spectroscopy using a DFB-QCL tuned over 673.8–675.1 cm− 1 and probing ro-vibrational transitions of the \(\nu\)4, \(\nu\)5, and \(\nu\)2 bands of C6H6, C2H2 (C-H bending), and CO2 (O-C-O bending), respectively. These species were carefully chosen to showcase the potential of laser-based sensing in the fingerprint region, as it is practically unfeasible to probe these species simultaneously using a single laser elsewhere in the IR region with reasonable sensitivity. Furthermore, the probed bands of C6H6 and C2H2 are the most intense bands of these species in the IR. There have been extensive efforts to measure these species using laser-based sensors by probing various spectral regions [13, 28, 36, 37]. Although this work is aimed at demonstrating a sensor at ambient conditions for air quality monitoring, the simultaneous detection of these species could be useful in several other applications; for example, time-resolved measurements of these species at high temperatures in combustion environments could be useful for the validation and refinement of chemical kinetic models [38, 39] and the investigation of soot formation phenomena. To our knowledge, this work represents the first demonstration of room-temperature, cw-laser-based interference-free sensing of benzene, acetylene, and carbon dioxide near in the long-wavelength mid-IR spectral region.