The effect of the number of parallel DNA molecules on electric charge transport through ‘standing DNA’

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Abstract
We present morphological and electrical characterization of double-stranded DNA (dsDNA) molecules covalently bound to two metal electrodes: an underlying gold surface and a gold nanoparticle (GNP). Conductive atomic force microscope (cAFM) with a metallized tip is used to perform current-voltage (I-V) measurements through dsDNA molecules, connected to GNPs of different diameters 5, 10 and 20 nm. The number of DNA molecules coating the GNP is expected to vary with the surface area of the GNP. This number and the portion of the GNP surface area enabling hybridization of the DNA determine the number of DNA molecules connecting the GNP to the gold surface. The larger the diameter of the GNP the higher the expected number of dsDNA molecules connecting it to the gold surface and thus the expected current. Our results show similar currents for all three GNP sizes, indicating that current flows through the same number of molecules regardless of the diameter of the measured GNP. The measured currents, 220 nA at 2 V, are in accordance with our previous reports (Cohen et al 2005 Proc. Natl Acad. Sci. USA 102 11589-93; Cohen et al 2006 Faraday Discuss. 131 367-76) in which we demonstrated the validity of the experimental system. In particular, for the 5 nm GNP, we conclude that the current possibly flows through two to three molecules, likely only one, and that a single short dsDNA molecule can support at least ∼70 nA, and probably 220 nA.