It is not through lack of trying. It is just very difficult. DNAs were evolved to work with proteins, or maybe it's the other way around – this is a separate question for biologists. For physicists, DNA is just a polymer that happens to carry the code of life. They firmly believe that there is no fundamental (physics) reason why DNA sequences cannot be read electronically using a nanopore.
Reporting their results in the journal Full detailsNanotechnology, researchers from Brown University led by physicist Xinsheng Sean Ling have revealed the progress that they have made in using a solid-state nanopore as a barcode scanner for reading out 12-mer "hybridization probes" on a DNA molecule. Since the exact sequences of the 12-mer "probes" are predetermined, it is not a big exaggeration to claim that they have sequenced a DNA, even though it is a very short one, 24 bases in total.
The Ling team has used synthetic oligonucleotides to make a model DNA consisting of three single-stranded DNA molecules (ssDNA). The three ssDNAs have matched ends and are hybridized together to form a single DNA comprising three single-stranded regions separated by two double-stranded segments, each of which is made of 12 bases. When this DNA molecule is pushed through a solid-state nanopore, two distinct dips in ionic current traces appear. Ling believes that this could be the first time that scientists have seen any sequence information from a nanopore experiment.
In principle, it should be possible to design a library of oligos and use nanopores to scan them to obtain DNA sequences. So has the Ling team won the race for nanopore DNA sequencing? Not yet. A quick calculation will show why. At each base, there are four possibilities A, C, G, T. To construct a library of 12-mer oligos, it will be necessary to scan through all 412=16777216 possible oligo probes. In other words, to completely sequence each DNA, more than 16 million samples have to be analysed.
The race is still on.