Ultra-sensitive sensors on a chip accurately detect trace gases
Traditional high-end gas sensors tend to be costly and large – typically containing a glass tube as their largest component, 100 microlitres in volume. However, smaller sensors can be unreliable if they do not use spectroscopy to detect specific gas molecules. “Our aim was to develop an ultra-sensitive trace gas sensor that can feel the tiniest ‘scent’ of molecules dispersed in the air,” explains project coordinator Jana Jágerská, associate professor at UiT The Arctic University of Norway(opens in new window) in Tromsø. “Traditional sensors are usually not specific enough for environmental applications. They suffer from calibration drift and other problems,” she notes. The sCENT(opens in new window) project eliminates the bulky gas tube, replacing it with a microchip combined with a miniaturised microfluidic cell of just 20 microlitres. The sCENT project was undertaken with the support of the European Research Council(opens in new window). The microfluidic top cell brings the gas sample to the chip’s surface for sensing. “The sample can be as small as a microlitre,” she says.
Using spectroscopy for accurate detection
Spectroscopy was combined with the project’s pioneering waveguide design for detecting specific gases. “Different absorption peaks in the spectra are uniquely linked to different chemical molecules,” explains Jágerská. “From the spectral position of those peaks, we know exactly what gases are in our sample, and from the depth of those peaks we know the concentration of those gases in the sample.” She adds that the sensor is very specific and does not require calibration, making it more reliable.
Challenge of moving to the more sensitive mid-infrared range
The project worked on various chip designs to integrate the waveguide photonics where guided light interacts extremely efficiently with the surrounding gases. But there were technical challenges, particularly in transitioning from the near-infrared to mid-infrared range – the range where the molecules have their spectral fingerprints. “If you want to do sensing sensitively, you go to the mid-infrared,” Jágerská remarks. “However, the mid-infrared spectral range turned out not to behave in the same way as [more common] near-infrared photonics.” “The materials used for near-infrared photonics at first appeared completely opaque in the mid-infrared range. So we had to work on modifying the designs and materials the chips were made of to get to the point of having a working device which we could then optimise further.”
Precision detection of trace gases
Detecting trace gases at high-precision parts-per-billion (ppb) levels is important for environmental monitoring and healthcare. The project focused on trace gases such as methane (CH4) and carbon dioxide (CO2), and then on detection of CO2 isotopes. “We could detect 300 molecules of methane in a billion of other molecules, an improvement of between one and two orders of magnitude compared to the state of the art,” Jágerská notes. “For CO2, we improved detection by four orders of magnitude to 20 ppb,” she adds, noting other research labs are typically unable to get below 100 parts-per-million (ppm). “This makes our sensors the best currently.” High sensitivity also aids CO2 isotope-specific detection. Isotopes can discriminate between man-made and biogenic emissions, and have applications in medical research. “Before, there were only high-end instruments for isotope detection. This is the first time it has been demonstrated on a photonic chip,” she says.
Microchip can be part of a monitoring device
The project went from “nothing but the ideas” to achieving proof of concept, according to Jágerská. The mid-infrared spectroscopic sensor integrated with a laser, detector chip and microfluidic cell can be incorporated into a device for environmental monitoring for example. “We are currently working on the assembly of the sensor into a prototype device,” she explains. This work is ongoing under the EU-funded sCENT2 project.
Keywords
sCENT, gas sensors, photonics, carbon dioxide, methane, trace gases, spectroscopy, near-infrared, mid-infrared, CO2 isotopes
