Tunable mid-infrared lasers in physical chemosensors towards the detection of physiologically relevant parameters in biofluids

Mid-infrared (mid-IR) laser spectroscopy is introduced as a physical chemosensor for liquids. Mid-IR sensor systems measure vibrational spectra of samples under investigation and thus are able to directly access chemical information. In this work, a pulsed External-Cavity Quantum-Cascade Laser (EC-QCL) with a 350 mW maximum emission power at 1180 cm−1 and tunable from 1030 cm−1 to 1230 cm−1, was used for transmission measurements of physiological solutions with pathlengths > 130 μm, thus significantly improving the measurement capabilities of mid-IR sensors employing thermal emitters. The first part of this paper discusses the characterization of the EC-QCL using time-resolved FTIR spectroscopy. Moreover, a comparison of the achievable performance of the physical chemosensor as opposed to a standard FTIR spectrometer is included. Results on the direct determination of glucose in spiked human serum in the physiologically relevant concentration range from 30 mg/dL to 400 mg/dL are reported. Furthermore, spectra of albumin, urea, lactate and phosphate in aqueous solution recorded with the EC-QCL chemosensor are shown, indicating its capability for simultaneous multi-analyte detection.