title: Where is the sodium in self-assembled monolayers of single-stranded DNA?
abstract: Monolayers of single-stranded DNA (ssDNA) immobilized on surfaces form the basis of a number of important biotechnology applications, including DNA microarrays and biosensors. The organization of ssDNA as layer on a solid substrate allows one to investigate various properties of the DNA in a controlled manner and to use DNA for analytical applications as well as for exploring futuristic schemes for molecular electronics. It is commonly assumed that the adsorbed DNA layer contains some structural water and the cations. Here we show, based on XPS studies, that when monolayers of ssDNA are formed from sodium phosphate buffer and washed thoroughly, no Na+ signal is detected. A finite concentration of ions is observed when the DNA is made from a solution of Mg2+ ions, but it is still only a fifth of what it would be if all the phosphate ions were fully neutralized by the metal cations.
authors.affiliation: Department of Chemical Physics and Chemical Research Support, Weizmann Institute, Rehovot 76100, Israel.
authors.forename: Supratim Guha
authors.initials: SG
authors.lastname: Ray
authors.forename: Hagai
authors.initials: H
authors.lastname: Cohen
authors.forename: Ron
authors.initials: R
authors.lastname: Naaman
authors.forename: Yitzhak
authors.initials: Y
authors.lastname: Rabin
mesh_terms: D004277:DNA, Single-Stranded / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D012964:Sodium / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: D004277:DNA, Single-Stranded; D012964:Sodium