We report a mid-IR transmission setup for the analysis of the protein amide I and amide II band in aqueous solutions that achieves a limit of detection as low as 0.0025 mg mL –1 (outperforming our previous results and other state-of-the-art mid-IR-based techniques by almost an order of magnitude). This large improvement is made possible by combining the latest-generation external cavity-quantum cascade laser (EC-QCL) operated at room temperature with an optimized double-beam optical setup that adjusts the path length (26 μm) to ensure robust sample handling. For minimizing the noise introduced by the high-intensity laser light source, a thermoelectrically cooled mercury cadmium telluride balanced detection module was employed. In this way, noise levels better by a factor of up to 20 were achieved compared with single-channel measurements. Characteristic spectral features of proteins with different secondary structures were successfully identified at concentrations as low as 0.1 mg mL –1 . Furthermore, a highly linear response was demonstrated for concentrations between 0.05 and 10 mg mL –1 . The total acquisition time of the setup can be adapted to fulfill the required sensitivity of the protein measurements and to ensure maximum flexibility for future applications. The presented setup combines high sensitivity, large optical path lengths, and short measurement times and thus outperforms previous research type EC-QCL setups as well as commercially available instruments. This opens a wide range of future applications including protein–ligand interaction studies as well as qualitative and quantitative analyses of proteins in complex matrices such as those found in up- and downstream bioprocess monitoring and similar challenging applications which can not be readily met by conventional FT-IR spectroscopy.

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In this paper, we present WaterSpy, a project developing an innovative, compact, cost-effective photonic device for pervasive water quality sensing, operating in the mid-IR spectral range. The approach combines the use of advanced Quantum Cascade Lasers (QCLs) employing the Vernier effect, used as light source, with novel, fibre-coupled, fast and sensitive Higher Operation Temperature (HOT) photodetectors, used as sensors. These will be complemented by optimised laser driving and detector electronics, laser modulation and signal conditioning technologies. The paper presents the WaterSpy concept, the requirements elicited, the preliminary architecture design of the device, the use cases in which it will be validated, while highlighting the innovative technologies that contribute to the advancement of the current state of the art.

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Most modern pulsed laser systems require versatile laser diode drivers. A state-of-the-art pulsed laser driver should provide precise peak power regulation, high repetition rate, and pulse duration control. A new, charge line dual-FET transistor circuit structure was developed to provide all these features. The pulsed modulation current is adjustable up to Imax = 1.2 A, with the laser diode forward voltage acceptable up to UF max = 20 V. The maximum repetition rate is limited by a charge line circuit to frep max = 20 MHz. Compared to the conventional single transistor drivers, the solution proposed in this paper allows a precise, high resolution width regulation to be obtained, whereas a low pulse jitter is ensured. In the solution, two separate, out-of-phase signals are used to trigger the individual Field Effect Transistors (FET). The resultant pulsed modulation current full-width-at-half-maxima (FWHM) is regulated from ~200 ps up to 2 ns. All control and timing signals are generated with a popular Field-Programmable Gate Array (FPGA) digital circuitry. The use of standard FPGA devices ensures the low cost and high reliability of the circuit, which are not available in laser drivers consisting of sophisticated analogue adjustable delay circuits.

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