The structure of the interface between a self-assembled monolayer (SAM) of alkanethiolates (AT) and the underlying Au(111) substrate is a longstanding puzzle in surface science. To cast further light on this problem, we took SAMs of hexanethiolate and dodecanethiolate on Au(111) as test systems and studied them by a combination of synchrotron-based high resolution X-ray photoelectron spectroscopy (HRXPS) and scanning tunneling microscopy (STM). The emphasis of the HRXPS characterization was put on the Au 4f emission of the substrate, which could be decomposed into the components related to the bulk and surface. The behavior of the surface component upon formation of hexanethiolate and dodecanethiolate SAMs was monitored in detail. We observed both a shift of this component and its branching into two independent emissions, which can be associated with two groups of Au atoms differently affected by the adsorption. This behavior, along with the relative intensity of both surface emissions, was correlated with most probable adsorption configurations of the thiolate headgroups. The analysis of the HRXPS data provides strong evidence for the involvement of the Au-adatoms, similar to the most recent models for short-chain AT monolayers on Au(111). The results indicate, however, that the structure of the long-chain systems is somewhat different and presumably more complex.

The structure and stability of single- and double-stranded DNA hybrids immobilized on gold are strongly affected by nucleotide-surface interactions. To systematically analyze the effects of these interactions, a set of model DNA hybrids was prepared in conformations that ranged from end-tethered double-stranded to directly adsorbed single-stranded (hairpins) and characterized by surface plasmon resonance (SPR) imaging, X-ray photoelectron spectroscopy (XPS), fluorescence microscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The stabilities of these hybrids were evaluated by exposure to a series of stringency rinses in solutions of successively lower ionic strength and by competitive hybridization experiments. In all cases, directly adsorbed DNA hybrids are found to be significantly less stable than either free or end-tethered hybrids. The surface-induced weakening and the associated asymmetry in hybridization responses of the two strands forming hairpin stems are most pronounced for single-stranded hairpins containing blocks of m adenine (A) nucleotides and n thymine (T) nucleotides, which have high and low affinity for gold surfaces, respectively. The results allow a qualitative scale of relative stabilities to be developed for DNA hybrids on surfaces. Additionally, the results suggest a route for selectively weakening portions of immobilized DNA hybrids and for introducing asymmetric hybridization responses by using sequence design to control nucleotide-surface interactions--a strategy that may be used in advanced biosensors and in switches or other active elements in DNA-based nanotechnology.

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