Study of the adsorption of sulfur-derivatized single stranded DNA on gold by atomic force microscopy and the cantilever bending technique

In this work, we study the role of the organosulfur linker in the adsorption of derivatized single stranded (ss) DNA on gold. The studied oligonucleotides are the unmodified ssDNA, the ssDNA modified with a thiol linker (SH-DNA), a symmetrical disulfide carrying two identical ssDNA molecules (DNA-SS-DNA) and an asymmetric disulfide carrying the protecting, group dimetoxitrityl (DNA-SS-DMTO). Surface inspection by atomic force microscopy shows that the modified oligonucleoticles and the non modified oligonucleoticles form monolayers on gold with similar structure and density. However, the results obtained by the cantilever bending method show that surface stress induced by the adsorption of ssDNA on gold critically depends on the organosulfur linker used. The results indicate that the surface stress measurement is mainly sensitive to the chemisorbed DNA molecules. We interpret the results using a two-step adsorption model, in which the oligonucleoticles quickly phisysorb on the gold surface before chemisorption is achieved via formation of the same alkanethiolate species in the three cases studied. Surface stress upon adsorption, and subsequent cantilever bending, is related to the transition between the physisorbed and chemisorbed states, in which the disulfide linker imposes a major difficulty to achieve the chemisorbed state due to geometrical and steric constraints.