Notes on Sample Binding for Fluid Imaging

Samples for AFM imaging should be immobilized on a rigid substrate. Macroscopic samples (such as certain 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, and DNA are usually bound to a flat substrate like mica or glass. 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 Bruker.

Binding Specimens to Mica

Mica is commonly used because atomically flat substrates can be simply and inexpensively prepared. In aqueous solutions, the cleaved mica 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 net positively charges. DNA 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⁺⁺).

Many other techniques are being developed for chemically modifying mica, glass and other substrates to bind a variety of biological samples. Contact Bruker for a bibliography of references on imaging of biological specimens.

Two procedures for binding samples to mica are described in this User Guide:

• Lysozyme on Mica—A Model Procedure for Protein Binding
• DNA on Mica—A Model Procedure for Nucleic Acid Binding

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