DNA
DNA is composed of four chemical bases: adenine, guanine, cytosine and thymine paired together in a complementary fashion and ordered in a species-specific sequence. The sequence represents a blueprint for the construction of the protein machinery to makes a cell work and store information. Sequencing of DNA involves cost and takes time because the procedure involves making multiple identical copies of the DNA and the chemistry involved.
Sequencing DNA with a nanopore is a revolutionary concept that may enable sequencing a single molecule of DNA, eliminating the need for amplification and for reducing the cost.
Nanopore
A nanopore is a hole made in a nanometre-thick thin membrane of materials such as silicon or silicon nitride. Nanopore is comparable in size to a DNA molecule.
Principle of DNA sequencing using nanopore
When the hole is immersed in electrolyte and a voltage applied across the membrane, a current flows through it. In addition to electrolytic ions, highly charged DNA molecules also try to electrophoretically migrate through the pore, but the hole is so small that only one molecule fits in the pore at a time.
By measuring either the voltage across the membrane or the current through the pore DNA is sequenced ,when each base pair as the DNA molecule translocates through the pore.
High-speed, high-fidelity sequencing can be done with low-noise electrical measurements at high frequency because the signal used to discriminate between bases is typically only at a microvolt or Pico amp level, and because the molecule translocates through the pore at a velocity of ~1 base pair per 10 ns.
Challenges
One of the main challenges is the signal-to-noise ratio required to discriminate between base pairs. The signal detection is limited by the bandwidth of the measurement and the corresponding electrical noise.
Researchers of University of Illinois, US, have reduced the high frequency noise in nanopores. With increased membrane thickness; miniaturized membrane area to reduce the parasitic capacitance; and by compensated membrane capacitance using an external electrical circuit the researchers have achieved this goal.
The reduction of the membrane capacitance is the key element to improving the overall electrical performance for sequencing DNA and the same has been proved by them by modeling the performance of its nanopore devices to show that further improvements in bandwidth and noise can be easily achieved by using thicker dielectric layers on top of the membrane.