Journal: medRxiv

Article Title: Intravital mid-infrared biosensing by normalized spatial probing of self-referenced optothermal signals

doi: 10.64898/2026.05.27.26354202

Figure Lengend Snippet: a, Working principle of Spatial Probing and Autocorrection Optothermal Spectroscopy (SPAC-OTHES). The system consists of a mid-infrared (MIR) quantum cascade laser (QCL), which irradiates a sample to induce the optothermal effect. The optothermal effect generates a heat field in the sample as well as the sample window, which is probed by a visible (VIS) laser. A camera and a diode assess the reflected beam from the VIS laser to quantify the intensity of the optothermal response and, thus, the MIR absorption in the sample. Depending on the signal acquisition delay, different depth slices can be probed. b , Signal detection linearity. The graph shows the areas under the curve of SPAC-OTHES intensity profiles of a water sample with varying laser power levels. c , Comparison of glucose absorption spectra retrieved by SPAC-OTHES with Fourier transform infrared spectroscopy (FTIR) spectra. d , SPAC-OTHES spectra of water-glucose phantoms with glucose concentrations varying from 0 to 1000 mg/dL. The graph shows the areas under the curve of SPAC-OTHES spectra. e , Comparison of lipid absorption spectra retrieved by SPAC-OTHES with FTIR spectra. f , Comparison of lactate absorption spectra retrieved by SPAC-OTHES with FTIR spectra. g , Comparison of albumin absorption spectra retrieved by SPAC-OTHES with FTIR spectra.

Article Snippet: The developed optothermal spectroscopy system uses a pump-and-probe detection principle, where an external cavity quantum cascade laser (EC-QCL Hedgehog, Daylight Solutions) with a tuning range from 8-10.6 μm is focused with a parabolic mirror (MPD169-M03, Thorlabs) and directed onto the sample for mid-IR excitation.

Techniques: Spectroscopy, Comparison, Fourier Transform Infrared Spectroscopy