Measurements performed by Stefan Kowalczyk, a graduate student in the lab of Cees Dekker at TU Delft in the Netherlands, show that previous models fail to describe the conductance of pores larger than about 15 nm in diameter. It turns out to be essential to include both the pore resistance, as well as the so-called access resistance, which refers to the resistance of the medium in a narrow region around, but not inside, the nanopore.

Hourglass geometry

The model is further improved by describing the nanopore not as a cylinder, but as an hourglass, which is much closer to the actual shape of the tiny holes revealed using electron microscopy. Theoreticians Alexander Grosberg and Yitzhak Rabin have worked out an exact solution for the conductance of such hourglass-shaped nanopores, which includes access resistance and was shown to be in excellent agreement with experimental data.

How does the conductance change upon insertion of a DNA molecule into the pore? Naively, one might expect this conductance change to be constant, independent of pore diameter, since the molecular volume that is blocking part of the pore is the same in all cases. However, surprisingly, experiments show that the conductance blockade increases as the pores become smaller. This effect can be understood by including the presence of the DNA in the access resistance region. The new models can form the foundation for understanding a multitude of conductance-based experiments on nanopores.

The researchers presented their results in the journal Nanotechnology.