Quantitative Analysis and Characterization of DNA Immobilized on Gold
MARYLAND UNIV COLLEGE PARK DEPT OF PHYSICS
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We describe the complementary use of X-ray photoelectron spectroscopy XPS and Fourier transform infrared FTIR spectroscopy to quantitatively characterize the immobilization of thiolated dT25 single-stranded DNA ssDNA on gold. When electron attenuation effects are accurately accounted for in the 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 chemically specific spectral signatures during immobilization indicates that at lower coverages much of the DNA lies flat on the surface, with a substantial fraction of the thymine bases chemisorbed. At higher immobilization densities, the dT25 film consists of randomly coiled ssDNA molecules each anchored via the thiol group and at possibly one or two other bases. We use two examples to demonstrate how the quantitative analysis can 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 immobilization and result in very high surface densities 5 x 10exp 13sq cm, densities that may only be possible if the divalent counterions induce strong attractive intermolecular interactions. In contrast with previous reports of alkanethiol adsorption kinetics on gold, ssDNA immobilization in 1 M phosphate buffer does not occur with Langmuir kinetics, a result attributable to rearrangement within the film that follows the initial adsorption.
- Inorganic Chemistry
- Atomic and Molecular Physics and Spectroscopy