  Photothermal-Induced Resonance (PTIR) spectroscopy and imaging with infrared
light has seen increasing application in molecular spectroscopy of biological
samples. The appeal of the technique lies in its capability to provide
information about IR light absorption at a spatial resolution better than
allowed by light diffraction, typically below 100 nm. In the present work we
test the capability of the technique to perform measurements with subcellular
resolution on intact eukaryotic cells, without drying or fixing. We demonstrate
the possibility to obtain PTIR images and spectra from the nucleus and multiple
organelles with high resolution. We obtain particularly strong signal from
bands typically assigned to acyl lipids and proteins. We also show that while a
stronger signal is obtained from some subcellular structures, other large
subcellular components provide a weaker or undetectable PTIR response. The
mechanism that underlies such variability in response is presently unclear. We
propose and discuss different possibilities, addressing thermomechanical,
geometrical and electrical properties of the sample and the presence of
cellular water, from which the difference in response may arise.
