Tools for the Characterization of Biomass at the Nanometer Scale - The Nanoscience and Technology of Renewable Biomaterials - page 80

3.4.4.2 Ultrafast Electron Microscopy

Nobel laureate Ahmed H. Zewail and associates developed ultrafast electron microscopy
(UEM) which enables imaging of kinetic processes at atomic resolution (168). A con-
ventional TEM was modified so that a femtosecond pulsed laser stimulated one electron
emission per laser pulse. A version capable of imaging wet specimens is under construc-
tion (169). Although the resolution is the same as for conventional TEM, the time resolu-
tion provided by this new technology promises to shed new light on dynamic processes.

3.5 Summary

Biomass is a difficult substrate to analyze at the nanoscale. It is a nanoscale, interpene-
trating, lightly crosslinked network of three different types of polymer. Compared with
most inorganics, the polymers of biomass are strongly affected by water, unstable, and
difficult to distinguish from each other.
Because the natural state of biomass is in water and because of the strong interaction
with water, native structures should be characterized in water when possible. Removing
water from biomass causes structural changes that are only partially reversible and not
well understood. The nanostructure of biomass will be different depending on the amount
of water present during analysis and how water has been removed (i.e. its history).
Because biomass is organic, many analytical techniques, particularly focused beam
microscopies, can easily damage specimens. Most particle or electromagnetic beams with
high resolution have the potential to vaporize or initiate chemical changes in the biomass
substrate. The probe beam can also cause accumulation of charge on the nonconducting
biomass substrate, which can directly effect measurements in some methods, as well as
cause further chemical changes.
Atomic force microscopes are the most common scanning probe microscopes, because
they are so versatile and easy to use. They were originally developed to measure
nanoscale topography, but a variety of techniques now provide information on chemical
and material properties as well. More specialized instruments, such as nanoindenters
and near-field scanning optical microscopes, may be less versatile but generally provide
'cleaner' information that requires the user to make fewer assumptions during analysis.
In short, the characterization of nanostructure in biomass is challenging. Like most
scientific problems, however, proper choice of analytical techniques and specimen prepa-
ration, as well as a skeptical approach to data interpretation, continue to provide new
understanding of this fascinating system.

References

1. Stamm A. Wood and Cellulose Science . New York: The Ronald Press Co 1964.

2. Berry S, Roderick M. Plant-water relations and the fibre saturation point. New

Phytologist . 2005;168:25-37.

3. Kellog RM, Wangaard FF. Variation in the cell wall density of wood. Wood and

Fiber . 1969;1:180-204.

4. Mantanis GI, Young RA, Rowell RM. Swelling of Wood, Part II Swelling in

Organic Liquids. Holzforschung . 1994;48(6):480-90.