Abstract
Scanning tunneling microscopy has found increasing interest in biological sciences. Several problems face the investigator imaging samples of biological origin. These include low conductivity, poor attachment of the sample to substrate and thermal/mechanical drift of the sample while scanning. Several approaches have tried to overcome these problems. These include coating with metal, freeze drying and/or freeze fracturing and tunneling in water. Another useful method is to freeze the sample on the STM holder and image frozen. The biological structure should stay rigid under the scanning tip, should not be swept away from the substrate and should provide highly reproducible images.Calf thymus DNA (Sigma #D-1501) was mixed in deionized water-tween 20 (0.1%). DNA concentrations varied between 10 μg/ml and 1 μg/ml. The sample was heated slightly, a small drop placed on highly oriented pyrolytic graphite (HOPG) and air dried. The sample holder was immediatel) placed in an Atomis variable temperature cryo-STM with a 6 micron scan range head (Surface Interface, Inc., Mountain View, CA), frozen and scanned.
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