This paper presents a study of sequence specific electronic conduction through short (15-base- pair) double-stranded (ds) DNA molecules, measured by immobilizing 3'-thiol-derivatized DNAs in nanometre scale gaps between gold electrodes. The polycation spermidine was used to stabilize the ds-DNA structure, allowing electrical measurements to be performed in a dry state. For specific sequences, the conductivity was observed to scale with the surface density of immobilized DNA, which can be controlled by the buffer concentration. A series of 15-base DNA oligonucleotide pairs, in which the centre sequence of five base pairs was changed from G: C to A: T pairs, has been studied. The conductivity per molecule is observed to decrease exponentially with the number of adjacent A: T pairs replacing G: C pairs, consistent with a barrier at the A: T sites. Conductance-based devices for short DNA sequences could provide sensing approaches with direct electrical readout, as well as label-free detection.
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