Optical photothermal infrared spectroscopy (O-PTIR): a promising new tool for bench-top analytical palaeontology at the sub-micron scale

The identification of preserved organic material within fossils is challenging. Well-established vibrational spectroscopy techniques, such as micro-FTIR (Fourier Transform Infra-Red spectroscopy), have been widely used to investigate organic fossils molecular composition. However, even when well-adapted to study objects several tens of micrometre across, they still suffer from limitations, notably regarding resolution and sample preparation requirements. Optical Photothermal Infrared Spectroscopy (O-PTIR), a recently developed technique, overcomes the challenges of bench-top FTIR spectroscopy. By combining an IR excitation laser with a 532 nm green probe laser, this technique allows molecular characterization at high spectral resolution (~2 cm-1) and with extremely fine spatial resolution (~500 nanometres). Additionally, problems linked with sample thickness, surface roughness and particle shape/size are mitigated when compared with FTIR or Atomic Force Microscopy-based nanoIR techniques. Here we show that O-PTIR can be used to easily and successfully map the molecular composition of small organic fossils preserved in silica matrix (chert) in petrographic thin sections. Our study reveals that O-PTIR resolves spatial heterogeneities in the preserved molecular composition of organic fossils (spores and plants) at a sub-micron scale, and that such heterogeneities occur in the cuticle in an early Devonian plant, where they suggest a structural organisation comparable to modern plants. These results on 400 million years old fossils, validate O-PTIR as a powerful and extremely promising new tool for nanoanalytical palaeontology.