Mid-IR Laser-Based Photothermal Spectroscopy: New Opportunities for Sensing and Imaging

Abstract

Advances in Instrumental Analytical Chemistry are often linked to technological developments in neighboring disciplines. This is the case with respect to recent advances in Mid-IR quantum cascade lasers (QCLs) which are increasingly used as a new light source in mid-IR spectroscopy. QCLs offer high spectral power densities, fast amplitude and frequency modulation possibilities, polarized and coherent radiation. Based on these properties a range of new sensing schemes, often clearly outperforming established FTIR based analyzers, have been developed recently. This presentation will center on photothermal sensing schemes for trace analysis of gases, liquids as well as label free mid-IR imaging with nanometer spatial resolution. As opposed to established mid-IR absorption spectroscopy based on Beer´s law, mid-IR photothermal spectroscopy is an indirect method where the generated analytical signal scales directly proportional to the laser power. In one way or another it detects temperature induced changes (refractive index changes, sample expansion) in the sample matrix which are caused by absorption of the mid-IR photons by the present analyte. After introducing the general concept of QCL based photothermal spectroscopy in comparison to absorption spectroscopy, applications will be shown covering different fields. For trace gas sensing interferometric cavity assisted photothermal spectroscopy (ICAPS) will be introduced [1]. This technique uses a Fabry-Perot interferometer to read out temperature induced refractive index changes in gaseous samples and achieves single digit ppb sensitivities for SO2, CO and similar IR active gases. ICAPS employs CW operated frequency tunable distributed feedback QCLs as an excitation source to target isolated ro-vibrational transitions of the target gas molecule and an NIR probe laser to monitor the induced refractive index changes. With respect to the analysis of liquids trace analysis of water in organic solvents will be shown and introduced as an alternative to Karl Fischer titration. This technique uses a broadly tunable pulsed external cavity QCL for measuring the characteristic bending vibration of condensed water. Again temperature induced refractive index changes are read out, this time using a HeNe based Mach Zehnder Interferometer with one arm containing the liquid sample in a flow cell. [2] Nanometer spatial resolution in mid-IR imaging is achieved by coupling an atomic force microscope to a broadly tunable pulsed EC-QCL source (AFM-IR). Using resonant enhancement and applying multivariate modelling the intracellular distribution of cell components can be revealed. This has been demonstrated by imaging the distribution of major cellulases and xylanases in Trichoderma reesei, using the co-location of a fluorescent label (enhanced yellow fluorescence protein, EYFP) with the target enzymes to calibrate the chemometric model. [3] The obtained PLS model successfully shows the distribution of these proteins inside the cell and opens the door for further studies on protein secretion mechanisms using AFM-IR.