  Atomic Force Microscopy - Infrared (AFM-IR) spectroscopy allows spectroscopic
studies in the mid-infrared spectral region with a spatial resolution better
than 50 nm. We show that the high spatial resolution can be used to perform
spectroscopic and imaging studies at the subcellular level in fixed eukaryotic
cells. We collect AFM-IR images of subcellular structures that include lipid
droplets, vesicles and cytoskeletal filaments, by relying on the intrinsic
contrast from IR light absorption. We also obtain AFM-IR absorption spectra of
individual structures. Most spectra show features that are recognizable in the
IR absorption spectra of cells and tissue obtained with FTIR technology,
including absorption bands characteristic of phospholipids and polypeptides. We
also observe sharp spectral features that we attribute to the nonlinear
photothermal response of the system and we propose that they can be used to
perform sensitive spectroscopy on the nanoscale.
