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SummaryDisassembly of DNA-ligands, including DNA-methylene blue (MB) complex and DNA-Co(phen) 3 3+ complex on mica surface, were investigated by atomic force microscopy (AFM). The disassembly of these complexes occurred after they were immersed in ultra-pure water. AFM results showed that the disassembly depended strongly on bridge ions that were used to immobilize the complex onto mica surface, DNA species and ligands. When Mg 2+ was used as the bridge ion, the DNA-MB complex was completely disassembled because of the weak interactions between Mg 2+ and DNA's bases or mica surface. Although if Co 2+ was used as the bridge ion, the disassembly of the DNA-MB complex mainly depended on the species and shape of DNA. For plasmid DNA pBR 322, plasmid DNA pUC 18 and the linear DNA pBR 322/PstI, the degree of disassembly was gradually increased. Whereas if Co(phen) 3 3+ was chosen as the ligand, the disassembly of the DNA-Co(phen) 3 3+ complex was almost blocked because Co(phen) 3 3+ could hardly diffuse into the ultra-pure water. This obtained information may be useful for practical application of the AFM imaging of biological molecules, especially in liquid.

The lambda-DNA molecules self-assemble on cysteamine-modified gold (111) surface to form flat-lying self-assembled monolayers (SAMs). The formation kinetics of such DNA SAMs is studied by atomic force microscopy (AFM). AFM results show that DNA molecules do not arrange themselves on cysteamine-modified gold (111) surface into a well-ordered monolayer. It is also found that the surface density of DNA monolayer does not increase as the DNA concentration increases. The high temperature of DNA solution and the immersing in ultrapure water produce some obvious DNA bundles. Whereas divalent cations in DNA solution result in the formation of more compact DNA films. The obtained information may be useful for practical application of the DNA films and further theoretical studies.