MASKING GENERATES CONTIGUOUS SEGMENTS OF METAL-COATED AND BARE DNA FOR SCANNING TUNNELING MICROSCOPE IMAGING

To date, no microscopic methods are available to confirm scanning tunn eling microscope (STM) images of DNA. The difficulties encountered in repeating these images may be attributed to inadequate distribution of molecules on the substrate, poor adhesion to the substrate, or the lo w conductivity of the molecules. However, these factors are difficult to assess in an STM experiment where they may act simultaneously. A me thod to isolate these factors involves partly masking the deposited mo lecules before coating them with a conductive film to produce adjacent segments of coated and bare DNA after the mask is removed. The coated DNA segments are conductive and mechanically stable to allow easy ide ntification of DNA by the STM. Furthermore, the path of a molecule can be traced from a coated to an uncoated region to test STM imaging of bare DNA. Masked preparations of DNA deposited on platinum/carbon-coat ed mica and highly oriented pyrolytic graphite were examined with a tu nneling current 1000 times lower than the usual nanoamps. The tip appa rently displaces molecules adsorbed to graphite to preclude imaging wh ereas more stably bound DNA on platinum/carbon-coated mica appears in reversed contrast.