  Rapid and cost-effective DNA sequencing at the single nucleotide level might
be achieved by measuring a transverse electronic current as single-stranded DNA
is pulled through a nano-sized pore. In order to enhance the electronic
coupling between the nucleotides and the electrodes and hence the current
signals, we employ a pair of single-walled close-ended (6,6) carbon nanotubes
(CNTs) as electrodes. We then investigate the electron transport properties of
nucleotides sandwiched between such electrodes by using first-principles
quantum transport theory. In particular we consider the extreme case where the
separation between the electrodes is the smallest possible that still allows
the DNA translocation. The benzene-like ring at the end cap of the CNT can
strongly couple with the nucleobases and therefore both reduce conformational
fluctuations and significantly improve the conductance. The optimal molecular
configurations, at which the nucleotides strongly couple to the CNTs, and which
yield the largest transmission, are first identified. Then the electronic
structures and the electron transport of these optimal configurations are
analyzed. The typical tunneling currents are of the order of 50 nA for voltages
up to 1 V. At higher bias, where resonant transport through the molecular
states is possible, the current is of the order of several $\mu$A. Below 1 V
the currents associated to the different nucleotides are consistently
distinguishable, with adenine having the largest current, guanine the
second-largest, cytosine the third and finally thymine the smallest. We further
calculate the transmission coefficient profiles as the nucleotides are dragged
along the DNA translocation path and investigate the effects of configurational
variations. Based on these results we propose a DNA sequencing protocol
combining three possible data analysis strategies.
