Techniques for the characterization of physical gels - Physical Gels from Biological and Synthetic Polymers - page 38

MgO labelled
gel surface

Quick frozen
gel block

Observation depth

5º

Figure 2.13

Surface scraping of a quick frozen gelatin block surface labelled with MgO crystals. Scraping
occurs obliquely with an inclination angle of 5°. Originally published by Favard et al.( 1989 )
© Portland Press.

investigations of their structure. The instrument is based on the scanning tunnelling
microscope, which was developed in the 1980s and provided the

first atomic resolution
images of surfaces. Since then, various developments have led to a group of related
microscopes, collectively known as the scanning probe microscope family. The funda-
mentals behind these microscopes are essentially the same.
A fine tip, usually c.20 nm and most commonly made from silicon nitride, is attached

to a spring-like device called a cantilever. The cantilever is also made from silicon and is
typically around 150 nm in length and 1 nm in thickness. When an image is sampled, the
probe or tip is lowered on to the sample surface, generating a force between the tip and
the sample which depends on the physical and chemical properties of the sample. As the
probe, controlled by a piezoelectric tube, moves along the surface of the sample, the
cantilever is de

ected.
Measurement of this cantilever de

ection is the key to obtaining topological informa-

tion. In early work this de of

electrons just above the cantilever, although soon afterward optical methods employing a
laser were developed. Using this method, de

ection was measured by the so-called ' tunnelling effect '

ections as low as 0.1 nm can be detected with
the laser and an appropriate optical detector. The detector is used to feed back information
to the equipment, which can use the data to control the vertical movement of the sample.
Two different techniques are currently used to obtain image information. In the

' , the distance between the sample and the tip is adjusted to keep the

cantilever at constant de

contact mode '

ection. The voltage supplied to the piezoelectric tube to

maintain constant de ection is used to provide an image. However, this technique has

proved to be unsuitable for imaging soft materials since it involves relatively high lateral
shear forces that can easily disrupt the sample.
Alternatively, and more widely, the so-called

' tapping ' or ' non-contact mode ' is used

to image fragile - including biological - materials. Here, the cantilever is oscillated in a