Tools for the Characterization of Biomass at the Nanometer Scale - The Nanoscience and Technology of Renewable Biomaterials - page 73

guide to AFM for organic and biological materials has been written by pioneers in this
field (92). This topic was written for biologists but contains detailed practical information
useful to chemists and material scientists working with soft and molecular materials.
A few examples of AFM studies of nanostructure in biomass are given here. Two
related papers treat the subject of specimen roughness and its effect on pull-off force
and the use of soft colloidal probes to minimize the effect of roughness on force mea-
surement (93, 94). The van der Waals forces between regenerated cellulose surfaces in
an aqueous environment with low pH and high ionic strength to suppress charge effects
is described by Notley (95), while Zauscher studied cellulose surface interactions in
various electrolyte concentrations (96).
A critical comparison of the AM (amplitude modulation) tapping mode and the FM
(frequency modulation) noncontact tapping mode for imaging soft matter is available
(97). In the FM mode, the tip exerts a very gentle force on soft materials and provides
height measurement close to the true value. In the AM mode, the tip typically exerts
a stronger force on soft materials and causes their deformation, especially in the liquid
environment.

3.4.2.2 Force Spectroscopy

In addition to the topographic image, force spectroscopy can provide chemical infor-
mation about a specimen from the forces that occur as the AFM probe approaches and
retracts from the surface (98, 99). Leite and Herrmann provide an introduction and
extensive review of this subject with a particular emphasis on adhesion phenomena
(100). Force curves can be recorded at many locations on the specimen surface. These
experiments can measure attractive and adhesion forces, the location and area of contact,
and the mechanical properties (modulus and plasticity) of the specimen.
Chemical force microscopy ( CFM ) uses chemically modified AFM tips to expand
the range of possible interactions. Once demonstrated (101), CFM has been applied to
characterize cell surfaces (102-104) and intermolecular interactions of cellulose surfaces
(96, 105). A thorough and readable introduction to CFM, along with many applications,
is suggested (106).
Nanotribology, or nanoscale friction, can also be studied using SPM (107, 108). An
in-depth review of friction force measurement applied mostly to atomic solids was pre-
pared by Carpick and Salmeron (109). The nonvertical component of stylus motion is
often attributed to friction or viscosity, but as Carpick points out, 'If the sample surface is
not flat, the surface normal force will have a component directed laterally and will result
in contrast in the lateral force image.' Despite these problems, valuable information on
friction at cellulose surfaces has been obtained with AFM (105, 110, 111).
A novel AFM design called torsional harmonic cantilever ( THC ) allows the mapping
of interaction forces and topology across a surface in a short time (112). Previous meth-
ods make time-consuming point-by-point measurements by approaching and retracting
the tip. By placing the tip off-center on the cantilever, every time the tip taps the sur-
face, the cantilever experiences a torque. The torsional oscillations are at a much higher
frequency than the flexural resonance of the cantilever, allowing them to be analyzed
separately to determine a force. As a demonstration of the capabilities of this approach,
Sahin et al . measured the mechanical properties of blend of poly(methyl methacrylate)