Magnetic AC mode atomic force microscopy (MAC Mode AFM) was used to characterize adsorption of DNA on a highly oriented pyrolytic graphite (HOPG) electrode surface using different concentrations of DNA and adsorption procedures. AFM of DNA immobilized on the HOPG showed that both single-stranded DNA and double-stranded DNA molecules have the tendency to spontaneously self assemble from solution onto the solid support and that this process is very fast. DNA condensed on the substrate in a tight and well spread two-dimensional lattice covering the entire surface uniformly. The interaction of DNA with the hydrophobic HOPG surface induced DNA superposition, overlapping, intra- and intermolecular interactions. Controlling the electrode potential offers the possibility to enlarge the capability of AFM imaging of DNA immobilised onto conducting substrates, such as HOPG. The effect of application of a positive potential of 300 mV (vs. Ag wire) to the HOPG electrode during adsorption was studied. The applied potential considerably enhanced the robustness and stability to mechanical stress of the DNA films, through multiple electrostatic interactions between the negatively charged hydrophilic sugar-phosphate backbone and positively charged carbon surface. The characteristics of the DNA films and the apparent height of the network wires were dependent on DNA concentration and immobilization procedure. The DNA lattices were held together on the substrate surface only by non-covalent interactions, such as hydrogen bonding, base stacking, electrostatic, van der Waals and hydrophobic interactions.

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