Plasmid DNA network on a mica substrate investigated by atomic force microscopy.

functional biopolymer. It has been widely used as a nanomaterial, such as DNA computer nanoinformation pits, DNA microarrays and DNA molecular conduction wires.1–3 Hence, it is important to fabricate a DNA nanoconstruction. Many research groups are attempting to fabricate DNA film, networks (monolayer), and other topological constructions. For example, the Seeman group reported the construction of a DNA (special single-strand DNA, well designed in base sequence) network: cube, Borromean rings and a truncated octahedron.1,2 Kanno et al. used a poly (dA-dT)·poly(dA-dT) DNA constructing large-scale network. Ye et al.4 made a DNA network by taking advantage of λ-bacteriophage DNA, which is a long and linear DNA molecule (48502 base pairs). Above them, all kinds of linear DNA were used to fabricate networks. It was not reported that a simple and easy construction method formed a DNA film and/or network uniformly over the entire substrate utilizing plasmid DNA. Even some groups thought that plasmid DNA could not form a DNA network, because plasmid DNA lacks the sticky ends.3–5 In this paper, we first report on the construction of a network using plasmid DNA. We adjusted the appropriate bivalence cationic concentration, Mg2+ or Ni(phen)3, to unfold the plasmid DNA pBR322 supercoils into rings. The extended DNA circles cross or overlapped each other, forming a network, owing to a bivalence cations bridge function. Moreover, our DNA network sufficiently covered the whole substrate. We propose the mechanism of plasmid DNA network formation and its model.