  We use photothermal induced resonance (PTIR) imaging and spectroscopy, in
resonant and non-resonant mode, to study the cytoplasmic membrane and surface
of intact cells. Non-resonant PTIR images apparently provide rich details of
the cell surface. However we show that non-resonant image contrast does not
arise from the infrared absorption of surface molecules and is instead
dominated by the mechanics of tip-sample contact. In contrast, spectra and
images of the cellular surface can be selectively obtained by tuning the
pulsing structure of the laser to restrict thermal wave penetration to the
surface layer. Resonant PTIR images reveal surface structures and domains that
range in size from about 20 nm to 1 um and are associated to the cytoplasmic
membrane and its proximity. Resonant PTIR spectra of the cell surface are
comparable to far-field IR spectra and provide the first selective measurement
of the IR absorption spectrum of the cellular membrane of an intact cell. In
resonant PTIR images, signal intensity, and therefore contrast, can be ascribed
to a variety of factors, including mechanical, thermodynamic and spectroscopic
properties of the cellular surface. While PTIR images are difficult to
interpret in terms of spectroscopic absorption, they are easy to collect and
provide unique contrast mechanisms without any exogenous labeling. As such they
provide a new paradigm in cellular imaging and membrane biology and can be used
to address a range of critical questions, from the nature of membrane lipid
domains to viral membrane fusion.
