Depth-selective photothermal IR spectroscopy of skin: potential application for non-invasive glucose measurement

An infrared spectroscopic technique is described that employs a mid-IR broadband (980-1245 cm^-1) tunable quantum cascade laser (QCL) to produce a pump beam, and a detection method based on photothermal deflection, enhanced by total internal reflection. The IR spectra thus obtained are depth-dependent by modulating the pump beam with different frequencies between 10 Hz and 500 Hz. A model system consisting of glucose and a polymer film is used to demonstrate the depth selectivity of this technique. We also apply this photothermal depth profiling method to record in vivo IR spectra of the human epidermis at different depths. This information can be used for a non-invasive glucose monitoring on diabetes patients, which is also demonstrated. Beyond biomedical infrared spectroscopy, there are numerous applications for total internal reflection enhanced photothermal deflection spectroscopy (TIR-PTDS). The high penetration depth of mid-IR light compared to the traditional ATR-FTIR technique and the easy sample access make this technique appropriate for in situ measurements, such as in industrial quality control. The depth selectivity of TIR-PTDS may be a convincing argument for its use in the analysis of multilayered samples or for the analysis of artwork, where the layers of interest are covered by a layer of varnish.