Orientation and Ordering in Monomolecular Films of Sulfur-Modified Homo-oligonucleotides on Gold

Abstract

The chemical integrity, packing density, orientation, and ordering in monomolecular films of sulfur-modified single-stranded DNA (ssDNA) on Au{111} were probed by a combination of X-ray photoelectron spectroscopy, angle-resolved near-edge absorption fine structure spectroscopy (at all relevant absorption edges), and infrared reflection absorption spectroscopy. As model systems, short chain (five nucleotides) thymine- and adenine-based homo-oligonucleotides were used. All ssDNA moieties were found to be bound to the substrate by the thiolate anchor, with no physisorbed ssDNA species but some contamination present. The density of the ssDNA moieties in the thymine- and adenine-based films was estimated at ∼4.8 × 1013 and ∼5.9 × 1013 mol/cm−2, respectively. At the same time, the former films exhibited a higher degree of the orientational order as compared to the latter ones. The lack of correlation between the packing density and orientational order is noteworthy and is assumed to be a specific property of sulfur-modified ssDNA films. Such behavior can be related to a special character of inter- and intramolecular interaction in these systems along with the influence of direct interaction between the nucleobases and substrate.