"With our fabrication and measurement techniques - micromoulding and four-point measurement of the electrical current - we have shown our ability to easily and reproducibly create artificial pores that can sense single molecules of lambda DNA," said Princeton researcher Lydia Sohn. "In addition, our technique allows us to create and measure simultaneously an array of eight parallel pores."

At the heart of each nanopore device was a 3 μm long, 200 nm diameter pore connecting two 5 μm deep reservoirs. To make the sensors, the scientists used electron-beam lithography and photolithography to fashion a negative master for the pore and reservoir structure. Then they poured PDMS over the master and cured it at 80°C for 24 hours. Finally, they removed the PDMS slab from the master and sealed it to a glass substrate containing platinum electrodes.

To detect the presence of molecules, the device uses the platinum electrodes to carry out a four-point measurement of the electrical current through the pore. When molecules are present, they partially block the flow of current, increasing the pore's electrical resistance. As a result, as molecules pass through the pore - driven by pressure applied to one of the reservoirs or by electrophoresis - the sensor exhibits temporary changes in resistance.

"Our work is the basis for a host of single-molecule sensing applications," explained Sohn. "By relating the measured change in electrical current to the length of each DNA molecule passing through the pore, we can use our nanopore device to coarse size large DNA molecules directly and quickly without fluorescent tagging."

Sohn also reckons that decreasing the size of the pores will allow the team to detect and size smaller molecules, such as proteins and viruses. "In addition, we can achieve chemical specificity by covalently attaching molecules of interest to the pore walls and seeing changes in the transit times of molecules in solution that interact with the immobilized molecules," she added.

The researchers reported their work in Nano Letters.