| Abstract
| - We describe the complementary use of X-ray photoelectron spectroscopy (XPS) and Fouriertransform infrared (FTIR) spectroscopy to quantitatively characterize the immobilization of thiolated (dT)25single-stranded DNA (ssDNA) on gold. When electron attenuation effects are accurately accounted for inthe XPS analysis, the relative coverage values obtained by the two methods are in excellent agreement,and the absolute coverage can be calculated on the basis of the XPS data. The evolution of chemicallyspecific spectral signatures during immobilization indicates that at lower coverages much of the DNA liesflat on the surface, with a substantial fraction of the thymine bases chemisorbed. At higher immobilizationdensities, the (dT)25 film consists of randomly coiled ssDNA molecules each anchored via the thiol groupand at possibly one or two other bases. We use two examples to demonstrate how the quantitative analysiscan be applied to practical problems: the effects of different buffer salts on the immobilization efficiency;the immobilization kinetics. Buffers with divalent salts dramatically increase the efficiency of immobilizationand result in very high surface densities (>5 × 1013/ cm2), densities that may only be possible if the divalentcounterions induce strong attractive intermolecular interactions. In contrast with previous reports ofalkanethiol adsorption kinetics on gold, ssDNA immobilization in 1 M phosphate buffer does not occur withLangmuir kinetics, a result attributable to rearrangement within the film that follows the initial adsorption.
|
