The use of synchrotron sources for infrared (IR) spectromicroscopy provides
greatly increased brightness that enables high-quality IR measurements at
diffraction-limited spatial resolutions. This capability permits synchrotro
n-based IR spectromicroscopy to be applied to biological applications at sp
atial resolutions on the order of the size of a single mammalian cell. The
question then arises, "Does the intense synchrotron beam harm biological sa
mples?" Mid-IR photons are too low in energy to break bonds directly; howev
er, they could cause damage to biological molecules due to heating. In this
work, we present measurements that show negligible sample heating effects
from a diffraction-limited synchrotron IR source. The sample used is fully
hydrated lipid bilayers composed of dipalmitoylphosphatidylcholine (DPPC),
which undergoes a phase transition from a gel into a liquid-crystalline sta
te at about 315 K during heating. Several IR-active vibrational modes clear
ly shift in frequency when the sample passes through the phase transition.
We calibrate and then use these shifting vibrational modes as an in situ te
mperature sensor.