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.
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