  High-spatial-resolution vibrational spectroscopy is one of the principal
techniques for nanoscale compositional analysis in biological materials. Here,
we present a new method for the analysis of whole-cell biological specimens
through nanoscale vibrational electron energy-loss spectroscopy (EELS) in the
monochromated scanning transmission electron microscope. Using the combined
spatial and spectral resolution of the technique, we examine the vascular
system of a cucumber stem and identify clear physical and vibrational
signatures from the different cellular regions with high spatial resolution.
Furthermore, using first-principles calculations combined with optical and EELS
spectroscopy on the individual components that make up the cucumber stem, we
unravel the physical mechanisms of the vibrational signatures and directly
assign compositional origins to the cell walls and bodies of different cellular
regions. These results demonstrate that monochromated electron energy-loss
spectroscopy is a promising technique for nanoscale spatial mapping of the
chemical composition of biological materials.
