Hydration scanning tunneling microscopy of DNA and a bacterial surface protein

Hydration scanning tunneling microscopy is based on the electrical conductivity of molecularly thin water layers which adsorb to the sample surfaces in a humid atmosphere. It allows reliable imaging of biological specimens and even insulators, provided they are hydrophilic. Here, we present results obtained with linearized plasmid DNA on mica and a bacterial surface protein layer (the HPI layer). A width of 3 nm was measured for the DNA molecules and a quasi-periodic structure along the molecules with a repeat distance of about 5 nm was observed. We show that-depending on the tunneling voltage-there are two different imaging modes for the DNA samples: at higher voltages, real tunneling or field emission is responsible for the charge transfer between tip and sample. In contrast, at lower voltages we found indications of a water meniscus between tip and surface. The HPI layer, however, seems to be imaged at most voltages without a water meniscus.