Using self-assembly to control the structure of DNA monolayers on gold: A neutron reflectivity study

Neutron reflectivity was used to determine the concentration profiles of oligomeric DNA monolayers on gold in high salt concentrations (1 M NaCl). These monolayers are of interest as models for DNA probe systems used in diagnostic devices. To facilitate its attachment, the DNA was functionalized at the 5' end with a thiol group connected to the oligonucleotide by a hexamethylene linker. Concentration profiles determined from neutron reflectivity indicate that adsorbed layers of single-stranded DNA (HS-ssDNA) on bare gold are compact, suggesting the presence of multiple contacts between each DNA strand and the surface. After treatment with mercaptohexanol, a short alkanethiol with a terminal hydroxy group, the DNA 'stands up' and extends farther into the solvent phase. These changes are consistent with the DNA remaining attached through its thiol end group while contacts between DNA backbones and the surface are prevented by the formation of a mercaptohexanol monolayer. The end-tethered HS-ssDNA layer readily hybridized to its complementary sequence, resulting in DNA helices with a preferred orientation toward the substrate normal.