Single molecule study of DNA conductivity in aqueous environment

The dc electrical conductivity of double stranded DNA is investigated experimentally. Single DNA molecules are manipulated with subpiconewton force and deposited on gold nanoelectrodes by optical traps. The DNA is modified at its ends for specific bead attachments and along the chain to favor charge transfer between the DNA base pair stack and the electrodes. For an electrode separation of $70\mathrm{nm}$ we find, in aqueous environment, electrical resistances above $100\mathrm{G}\Omega$ indicating that even for weak stretching the double helix is almost insulating at this length scale.

Figure 1

Principle of the experiment. A single DNA molecule is precisely positioned by optical trap micromanipulation on electrodes under liquid, thus allowing for direct electrical measurements. Disulfide groups are introduced along the chain to enhance the electronic coupling to the electrodes.

Figure 2

Scanning electron microscopy (SEM) images of the gold nanoelectrodes on oxidized silicon. The bars indicate the scales. The $160\mathrm{nm}$ wide electrodes are separated by $70\mathrm{nm}$ gaps (see Ref. 13).

Figure 3

Force vs extension curve of a single DNA molecule in aqueous environment, measured with the double optical trap. The dashed line shows a calculation based on a modified Marko-Siggia wormlike chain model (see Ref. 15), using the following fit parameters: a contour length of $3.5\mu \mathrm{m}$, a persistance length of $50\mathrm{nm}$, and an elastic modulus of $1000\mathrm{pN}$.

Figure 4

Optical microscope images of the deposition of a molecule linking two beads on the electrodes. The beads are (a) laterally positioned above the electrodes which appear unfocused, then (b) down translated to the surface. Because the beads are trapped and imaged by the same objective, they are always in focus.

Figure 5

Two-contact $I\left(V\right)$ characteristics measured at $0.01\mathrm{Hz}$ in aqueous environment for the sample before and after deposition of a DNA molecule. The buffer is either PB $1\times$ or PB $1\times$ with $1\mathrm{mM}$ $\mathrm{Mg}{\mathrm{Cl}}_2$. Current modulus (bottom) and current phase (top) are presented.