Infrared Nano-Spectroscopy of Single Cell Sections at the Ultrastructural Level.

Resin-embedded cell sections are the standard approach for ultrastructural imaging with electron microscopy. Recently, infrared (IR) nanoimaging has emerged as a label-free alternative that visualizes cellular structure based on intrinsic molecular contrast. However, assignment of cellular components still requires morphological interpretation. Here, we demonstrate that nano-FTIR spectroscopy can chemically distinguish subcellular structures by their IR spectral fingerprint. To achieve this, we implemented interleaved referencing to mitigate interferometer drift between sample and reference measurement, reaching a near-field phase sensitivity of 4 mrad. Combined with an effective medium model to remove resin-derived IR absorption bands, our approach enables reliable detection of small differences in cellular IR absorption with 50 nm spatial resolution. Using human cervical cancer (HeLa) and mouse osteoblast (MC3T3) cell lines as examples, we spatially resolved intracellular variations in protein and nucleic acid (DNA/RNA)-associated IR bands. Nucleoli were identified as regions with the highest molecular content within the nucleus, whereas cytoplasm showed cell-type-dependent differences: elevated nucleic acid bands in HeLa and reduced in MC3T3 cells. This direct approach for molecular mapping offers opportunities for biomedical research, providing multichannel IR information on individual cell organelles that can be analyzed for understanding cell organization, disease mechanisms, drug delivery and nanoparticle treatments.