Fluid Operation
General Notes on Sample Binding
Rev. B
MultiMode SPM Instruction Manual
141
8.5
General Notes on Sample Binding
Samples for AFM imaging should be immobilized on a rigid substrate. Macroscopic samples
(biomaterials, crystals, polymer membranes, etc.) can be attached directly to a stainless steel
sample disk with an adhesive. Dissolved or suspended samples like cells, proteins, DNA, etc. are
usually bound to a
fl
at substrate like mica or glass, for example. Many different sample preparations
have been developed and SPM applications articles are an excellent source of information on
sample binding. For a list of articles describing biological applications of AFM, including sample
preparation techniques, contact Veeco.
Binding Specimens to Mica
The following procedures will describe how to bind two different samples to a mica substrate. Mica
is commonly used because atomically
fl
at substrates can be simply and inexpensively prepared. In
aqueous solutions, the mica cleavage surface becomes negatively charged. Specimen binding is
usually accomplished using electrostatic attraction between charges on the specimen and those on
the mica surface. Proteins, for example, can usually be made to stick to mica by operating at a pH
where they exhibit positively charged domains. DNA, on the other hand, is negatively charged and
can be bound either by altering the mica surface charge from negative to positive (using a
silanization process) or by dissolving the DNA in a divalent metal counter ion (e.g. Mg
++
, Ni
++
).
Both of these techniques are discussed separately below.
Many other techniques are being developed for chemically modifying mica, glass and other
substrates to bind a variety of biological samples. Contact Veeco for a bibliography of references on
imaging of biological specimens.
