Penn Researchers Control the Size of 2-D Nanopores With Light

Researchers at the University of Pennsylvania are investigating a new technology that, if proven, could lead to small, chip-size sensors capable of sensing molecules and detecting illnesses or even possibly the presence of viruses.

The technology is focused on nanopores, which are tiny holes typically just a few nanometers across, 100,000 times smaller than the width of a single strand of human hair. Threading a strand of DNA, which in its single-strand form has a diameter of 1.1 nanometers, through these nanopores allows scientists to sequence the bases of the DNA due to their different sizes.

Around 2008, researchers had the idea of creating these nanopores in two-dimensional materials such as graphene, which is a single-atom thin membrane. But defects and certain properties of these materials can cause issues in DNA sequencing.

Now, researchers at Penn and Pennsylvania State University, as part of a joint grant from the National Science Foundation, are testing nanopores in a novel 2-D material called tungsten disulphide, which may provide advantages over graphene, including exploiting the optical properties of this class of metal dichalcogenide materials. In addition to realizing nanopores in this material and proving that these nanopores work in salt solution, the researchers found a surprising twist, the nanopores responded to light, expanding when laser light was shined on them, with an expansion rate that increased as the laser power was increased.

“What I was expecting was that by shining light, we could affect some aspect of DNA motion through the pore,” said Marija Drndić, the Fay R. and Eugene L. Langberg Professor of Physics in the School of Arts & Sciences at Penn. “But when we shined the light, we saw that the pores expanded.”

The research was led by Drndić and Penn graduate students Gopinath Danda and Paul Masih Das. A.T. Charlie Johnson, a professor of physics in Penn’s School of Arts & Sciences and director of the Nano/Bio Interface Center at Penn, and Mauricio Terrones, a professor of physics at Penn State, also contributed to the research.

Not only is this the first time research has been done on nanopores in this particular material, but it is also one of the first times researchers have studied how nanopores react to light.

“To our knowledge,” Masih Das said, “we were the first people to work with this material and design it with this optical focus. Usually people aim to create a nanopore capable of getting the highest resolution, but they don't think about using the properties of the material itself.”

​​​​​​​The researchers grew the tungsten disulfide in a chemical vapor deposition chamber, where they flowed gases onto a substrate to grow the material in a 2-D fashion. They then peeled off the material, applied it to their membranes and drilled a tiny hole into it with an electron beam. Then they immersed the material in a salt solution and applied a voltage to drive the charged DNA molecules to flow through the nanopores.

While the material was in the solution, they shined laser light on it and studied the effects.

Although the researchers hadn’t expected the pores to expand under light, they had chosen the material due to its interesting optical properties. They found that the laser light was triggering a chemical reaction so that the light-facilitated interaction between the material and the salt solution around it caused pores to grow.

It’s possible that this finding may provide researchers with a faster, less expensive technique for making and growing nanopores. If there’s a tiny defect in the material and they shine light on it, it may provide a starting point for pore formation and growth.

“We'd like to see if we can use this phenomenon, this chemical reaction, to actually make pores with light,” Drndić said. “That's one direction that could really benefit this whole technology because then you can imagine shining light and creating pores, which would be very fast and inexpensive.”

​​​​​​​Although nanopore technology is still in its developmental stage, the researchers hope it can be used for DNA or protein sequencing, biomarker detection and the quick detection of illnesses.

“Frequently we learn about some illnesses once they spread enough that you see symptoms,” Drndić said. “But if you could detect the presence of viruses quickly by looking at their DNA, then you could map out how things are spreading.”

One benefit of the technology is that it can work with small quantities of molecules since it detects one molecule at a time. It also works through electronic signals, which means that the entire device could operate at high speed and fit into a small electronic chip.

The researchers hope to harness this phenomenon to create nanopore sensors with dynamic size control.

​​​​​​​“We can use the same nanopore device,” Danda said, “to separate a solution of particles of different sizes because we can control the nanopore size.”

This would allow them to analyze biomolecules such as DNA and viruses using just one device by starting with a small nanopore, allowing all the small molecules to pass through it and then using light to expand the nanopore and pass bigger particles through.

“As scientists at a university, we have the luxury of playing around a little bit to see what works better,” Drndić said. “We’re able to explore, which can lead to some new ideas. We’re laying down the fundamental basis behind how nanopores work so that one day engineers can continue the technology.”