  Single molecule force spectroscopy of DNA strands adsorbed at surfaces is a
powerful technique used in air or liquid environments to quantify their
mechanical properties. Although the force responses are limited to unfolding
events so far, single base detection might be possible in more drastic
cleanliness conditions such as ultra high vacuum. Here, we report on high
resolution imaging and pulling attempts at low temperature (5K) of a single
strand DNA (ssDNA) molecules composed of 20 cytosine bases adsorbed on Au(111)
by scanning probe microscopy and numerical calculations. Using electrospray
deposition technique, the ssDNA were successfully transferred from solution
onto a surface kept in ultra high vacuum. Real space characterizations reveal
that the ssDNA have an amorphous structure on gold in agreement with numerical
calculations. Subsequent substrate annealing promotes the desorption of solvent
molecules, DNA as individual molecules as well as the formation of DNA self
assemblies. Furthermore, pulling experiments by force spectroscopy have been
conducted to measure the mechanical response of the ssDNA while detaching. A
periodic pattern of 0.2 to 0.3nm is observed in the force curve which arises
from the stick slip of single nucleotide bases over the gold. Although an intra
molecular response is obtained in the force curve, a clear distinction of each
nucleotide detachment is not possible due the complex structure of ssDNA
adsorbed on gold.
