Biomedical Engineering Reference
In-Depth Information
non-toxic and there are a wide range of methods available to produce highly pure water.
It is always worth remembering that not all water is the same, however, so for samples
with very small
z
-heights (e.g. nanoparticles
20 nm, proteins, nucleic acids), very pure
water is required, in order that the contaminants do not mask the sample. Examples of the
effects of differing water grades are shown in Figure 4.2. If very clean solution cannot
be found, it may be advantageous to find a way to adhere the particles to the surface, which
could be via silane or polycation modification [287-289], followed by washing, although
this has the disadvantage of increasing substrate roughness [290]. Substrates for AFM are
discussed in Section 4.1.1.
If it is desired to image a dry powder without dissolving in a liquid, a number of
techniques have been described to immobilize particles. One technique is to immobilize
large particles in a filter or similar porous substrate [291]. This can lead to the particles
being sufficiently fixed to be able to scan them, and the top of the particle will be
available to scan, but the full height of the particles will not be measurable. An
alternative is to scatter the powder on an adhesive surface, such as a flat substrate
with a thin layer of glue. Ideally the glue will be cross-linked/dried after the powder is
applied, to avoid contamination of the AFM tip. Other systems that can work well with
such samples include poly-l-lysine coated glass, and thin layers of wax, to which the
sample is applied while the wax is soft (at elevated temperatures), and which solidifies
on cooling [286]. For very small particles (
<
20 nm), many chemical modifications of
the substrate surface produce a surface that is too rough for quantitative measurements.
In such cases, deposition from ultra-pure water onto mica or HOPG is the best
technique. Alternatively, some mica treatments have been described that increase the
roughness only slightly [290].
<
Polymers
Polymer samples come in a wide variety of forms. Solid samples may require no
preparation other than cutting to size and cleaning. Preparation of polymer films for
AFM is also simple, and may be done by casting, spin coating, spreading, self-assembly,
dip coating etc [146, 292, 293]. Typically such films are deposited on glass slides, as there
is no requirement for very flat substrates.
Fig. 4.2. Example of the importance of clean solvent: images of a very flat substrate (mica) after
deposition of drops of 'pure' water, followed by drying. Left: tap water. Middle: deionized, filtered
water. Right: commercial ultra-pure water. All images are 2
m
2
m,
z
-scale 4 nm.
