[Zurück]

A. Dabrowska, M. David, A. Schwaighofer, S. Freitag, A. M. Andrews, G. Strasser, B. Hinkov, B. Lendl:
"Broadband Mid-Infrared Sensor employing a Quantum Cascade Laser and a Quantum Cascade Detector for Milk Protein Analysis";
Vortrag: ICAVS 11, Poland; 23.08.2021 - 26.08.2021.

Recent advances of quantum cascade technologies in the mid-infrared region enable new spectroscopic sensing schemes. Novel laser light sources, such as external cavity quantum cascade lasers (EC-QCLs) are gaining more attention for qualitative and quantitative analysis of liquids as they can outperform conventional FTIR spectrometers in terms of sensitivity, portability and costs [1].
Accompanied by developments in QCLs, there has also been progress in quantum engineering-based detectors. Quantum cascade detectors (QCDs) offer room-temperature operation, fast response times, low noise and high potential for integration. It was demonstrated that QCDs monolithically integrated with QCLs enable the development of miniaturized and cost-effective mid-IR sensing devices [2,3]. In contrast to HgCdTe (MCT) detectors, widely employed in FTIR spectrometers and laser-based spectroscopy setups, QCDs operate in a wide power range of the incident radiation without saturation effects while maintaining excellent linearity, crucial for reliable quantitative spectroscopy. Hence, no additional optical components, i.e. band-pass or neutral density filters, have to be used to reduce the intensity of the laser beam to remain in the linear range of the detector [1]. Even though QCDs offer many favorable properties for spectroscopic applications, only a few examples of their application in spectroscopic systems were demonstrated so far and their use predominantly focused on gas-phase analysis.
In the presented work, we combine a broadly tunable QCL and a QCD for broadband liquid-phase spectroscopy for sensitive and selective detection of bovine milk proteins. A thermoelectrically-cooled EC-QCL (DRS Daylight Solutions) tunable from 1730 to 1470 cm-1 was incorporated in the setup. A custom-made temperature-stabilized transmission flow cell with a 12.5 μm PTFE spacer was used for liquid sample handling. For signal detection, a ridge QCD was used and operated at room-temperature (298 K). The spectral response of the QCD employed in the setup overlaps well with the tuning range of the laser, allowing detection of the two most prominent absorption bands of proteins (amide I and amide II). Broadband infrared spectra of bovine milk proteins (casein, β-lactoglobulin, α-lactalbumin) dissolved in buffer solution at multiple concentrations ranging from 0.25 - 15 g L-1 were recorded. A comparison to FTIR absorbance spectra shows excellent agreement. The root-mean square noise level obtained by the setup was of 0.067 mAU and the limit of detection for milk protein sensing was found to be at ~0.09 g L-1.
In summary, this is the first demonstration of the use of a QCD in combination with an EC-QCL for broadband IR spectroscopy of liquid-phase samples. We showcased that a spectrometer fully based on quantum cascade technologies for mid-IR light generation and detection can be successfully used for broadband spectroscopy of liquids, e.g. milk proteins, achieving similar performance as a high-end FTIR spectrometer, while maintaining much greater compactness and simplicity.

quantum cascade detector, quantum cascade laser, mid-infrared spectroscopy, physical chemosensors, milk protein analysis