Abstract
Mid-infrared chemical sensors based on quantum cascade technology offer a number of properties surpassing conventional spectrometric techniques. In this work, we combine a tunable quantum cascade laser with a spectrally tailored in-house fabricated quantum cascade detector (QCD) to realize broadband detection of aqueous samples for selective sensing of bovine milk proteins. The developed setup enables broadband spectroscopy covering more than 260 cm−1 and was employed to record absorbance spectra of the amide I and amide II bands of β-lactoglobulin, α-lactalbumin and casein. A detailed comparison indicates similar performance of the laser-based setup with its uncooled QCD as a high-end FTIR spectrometer equipped with a liquid nitrogen cooled mercury-cadmium-telluride (MCT) detector. Furthermore, we discuss the characteristics and benefits of the quantum cascade detector for application in laser-based mid-infrared sensor systems and compare its performance to other common mid-infrared detector types. In conclusion, the combination of QCDs with EC-QCLs opens up new possibilities for next-generation MIR liquid-phase chemical sensors featuring low noise and high dynamic range.
Highlights
Mid-infrared (MIR) spectroscopy (4000 – 400 cm− 1 / 2.5 – 25 μm) allows for highly selective, sensitive, and non-destructive sample investigation by probing molecular vibrations
We demonstrated the first use of a quantum cascade detector (QCD) in combination with an external cavity (EC-)quantum cascade lasers (QCLs) for broadband MIR spectroscopy of liquid-phase sam ples
We further demonstrated that our EC-QCL-QCD sensor system has equal performance for protein analysis as high-end Fourier transform infrared (FTIR) spectroscopy, while maintaining much greater compactness and simplicity
Summary
Mid-infrared (MIR) spectroscopy (4000 – 400 cm− 1 / 2.5 – 25 μm) allows for highly selective, sensitive, and non-destructive sample investigation by probing molecular vibrations. Fourier transform infrared (FTIR) spectroscopy incorporating thermal light sources (i.e. Globars) is considered the gold standard Employing this technique makes the analysis of protein samples usually a cumbersome task because of the strong water absorption band near 1645 cm− 1, overlapping with the amide I band and signifi cantly absorbing the radiation of a weak thermal light source (μW/cm− 1). This limits the applicable path lengths for transmission measurements to typically 5–8 μm, impairing the robustness of liquid sample handling and complicating measurements of complex matrices such as milk [1]. In commercial FTIR-based products, larger path length cells are in use (~35 μm), allowing protein analysis only in the amide II band region and limiting information on the protein secondary structure
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